Multi-layer, microporous membrane, battery separator and battery

ABSTRACT

The invention relates to a multi-layer, microporous membrane having appropriate permeability, pin puncture strength, shutdown temperature, shutdown speed, meltdown temperature, and thickness uniformity. The invention also relates to a battery separator formed by such multi-layer, microporous membrane, and a battery comprising such a separator. Another aspect of the invention relates to a method for making the multi-layer, microporous polyolefin membrane, a method for making a battery using such a membrane as a separator, and a method for using such a battery.

FIELD OF THE INVENTION

The invention relates to a multi-layer, microporous membrane havingappropriate permeability, pin puncture strength, shutdown temperature,shutdown speed, meltdown temperature, and thickness uniformity. Theinvention also relates to a battery separator formed by suchmulti-layer, microporous membrane, and a battery comprising such aseparator. Another aspect of the invention relates to a method formaking the multi-layer, microporous polyolefin membrane, a method formaking a battery using such a membrane as a separator, and a method forusing such a battery.

BACKGROUND OF THE INVENTION

Microporous polyolefin membranes can be used as battery separators in,e.g., primary and secondary lithium batteries, lithium polymerbatteries, nickel-hydrogen batteries, nickel-cadmium batteries,nickel-zinc batteries, silver-zinc secondary batteries, etc. Whenmicroporous polyolefin membranes are used for battery separators,particularly lithium ion battery separators, the membranes' performancesignificantly affects the properties, productivity and safety of thebatteries. Accordingly, the microporous polyolefin membrane should havesuitable mechanical properties, heat resistance, permeability,dimensional stability, shutdown properties, meltdown properties, etc. Asis known, it is desirable for the batteries to have a relatively lowshutdown temperature and a relatively high meltdown temperature forimproved battery-safety properties, particularly for batteries that areexposed to high temperatures during manufacturing, charging,re-charging, use, and/or storage. Improving separator permeabilitygenerally leads to an improvement in the battery's storage capacity.High shutdown speed is desired for improved battery safety, particularlywhen the battery is operated under overcharge conditions. Improving pinpuncture strength is desired because roughness of the battery'selectrode can puncture the separator during manufacturing leading to ashort circuit. Improved thickness uniformity is desired becausethickness variations lead to manufacturing difficulties when winding thefilm on a core. Thickness variations can also lead to non-isotropictemperature variations in the battery, which can lead to batteryhot-spots (regions of higher temperature) where the separator isrelatively thin.

In general, microporous membranes containing polyethylene only (i.e.,the membrane consists of, or consists essentially of, polyethylene) havelow meltdown temperatures, while microporous membranes containingpolypropylene only have high shutdown temperatures. Accordingly,microporous membranes comprising polyethylene and polypropylene as maincomponents have been proposed as improved battery separators. It istherefore desired to provide microporous membranes formed frompolyethylene resin and polypropylene resin, and multi-layer, microporousmembranes comprising polyethylene and polypropylene.

JP7-216118A, for example, discloses a battery separator having asuitable shutdown temperature and mechanical strength. The patentpublication discloses a battery separator comprising a multi-layer,porous film having two microporous layers. Both layers can containpolyethylene and polypropylene, but in different relative amounts. Forexample, the percentage of the polyethylene is 0 wt.% to 20 wt.% in thefirst microporous layer, and 21 wt.% to 60 wt.% in the secondmicroporous layer, based on the combined weight of the polyethylene andpolypropylene. The total amount of polyethylene in the film (i.e., bothmicroporous layers) is 2 wt.% to 40 wt.%, based on the weight of themulti-layer microporous film.

JP10-195215A discloses a relatively thin battery separator havingacceptable shutdown and pin-pulling characteristics. The term “pinpulling” refers to the relative ease of pulling a metal pin from alaminate of a separator, a cathode sheet and an anode sheet, which iswound around the pin, to form a toroidal laminate. The multi-layer,porous film contains polyethylene and polypropylene, but in differentrelative amounts. The percentage of polyethylene is 0 wt. % to 20 wt. %in the inner layer and 61 wt. % to 100 wt. % in the outer layer, basedon the total weight of the polyethylene and polypropylene.

JP10-279718A discloses a separator designed to prevent unacceptablylarge temperature increases in a lithium battery when the battery isovercharged. The separator is formed from a multi-layer, porous filmmade of polyethylene and polypropylene, with different relative amountsof polyethylene and polypropylene in each layer. The film has apolyethylene-poor layer whose polyethylene content is 0 wt. % to 20 wt.%, based on the weight of the polyethylene-poor layer. The second layeris a polyethylene-rich layer which contains 0.5 wt. % or more ofpolyethylene having a melt index of 3 or more and has a polyethylenecontent of 61 wt. % to 100 wt. %, based on the weight of thepolyethylene-rich layer.

It would be desirable to further improve the permeability, pin puncturestrength, and shutdown speed of microporous polyolefin membranes.Moreover, it would be desirable to further improve the thicknessuniformity of microporous polyolefin membranes in order to reduce thelikelihood of short-circuiting when used as battery separators.

SUMMARY OF THE INVENTION

In an embodiment, the invention relates to a multi-layer membrane,comprising: a first layer material comprising a first polyethylene and afirst polypropylene and a second layer material comprising a secondpolyethylene and a second polypropylene, the second polypropylene havinga weight-average molecular weight of 6.5×10⁵ or more and a heat offusion of 95 J/g or more, the fraction of the second polypropylenehaving a molecular weight of 1.8×10⁶ or more being 10% or more by massby mass of the second polypropylene. The multi-layer membrane cancomprises e.g., a first microporous layer containing the firstmicroporous layer material and a second microporous layer containing thesecond microporous layer material. For example, the multi-layer membraneof claim 1 can comprise:

-   -   a first microporous layer containing the first microporous layer        material, a third microporous layer containing the first        microporous layer material, and a second microporous layer        containing the second microporous layer material, the second        microporous layer being located between the first and third        microporous layers.

In an alternative form, the multi-layer membrane can comprise:

-   -   a first microporous layer containing the second microporous        layer material, a third microporous layer containing the second        microporous layer material, and a second microporous layer        containing the first microporous layer material, the second        microporous layer being located between the first and third        microporous layers.

In another embodiment, the invention relates to a method for producing amicroporous membrane, comprising,

-   -   (1) combining a first polyethylene resin, a first polypropylene        resin, and a first process solvent to form a first polyolefin        solution, wherein the first polyethylene resin and the first        polypropylene resin together constitute a first polyolefin        composition; and wherein the amount of the first polyethylene        resin in the first polyolefin composition is at least about 80        wt. %, based on the weight of the first polyolefin composition;        and    -   (2) combining a second polyethylene resin, a second        polypropylene resin, and a second process solvent to form a        second polyolefin solution, wherein the second polyethylene        resin and the second polypropylene resin together constitute a        second polyolefin composition; and wherein the amount of the        second polyethylene resin in the second polyolefin composition        is at least about 50 wt. %, based on the weight of the second        polyolefin composition the second polypropylene resin having a        weight-average molecular weight of 6.5×10⁵ or more and a heat of        fusion of 95 J/g or more, the fraction of the second        polypropylene resin having a molecular weight of 1.8×10⁶ or more        being 10% or more by mass by mass of the second polypropylene        resin. In an embodiment, the invention further comprises    -   (3) extruding at least a portion of the first polyolefin        solution through a die or dies and co-extruding at least a        portion of the second polyolefin solution in order to form a        multi-layer extrudate,    -   (4) cooling the multi-layer extrudate to form a multi-layer        sheet,    -   (5) removing at least a portion of the process solvents from the        multi-layer, sheet to form a solvent-removed sheet, and    -   (6) removing at least a portion of any volatile species from the        sheet to form the multi-layer, microporous membrane.

In yet another embodiment, the invention relates to a battery comprisingan anode, a cathode, and electrolyte, and the multi-layer membrane ofthe preceding embodiments, wherein the multi-layer membrane separates atleast the anode from the cathode. The battery can be used as a source orsink of electric charge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing one example of typical DSC curves.

FIG. 2 is a graph showing another example of typical GPC curves.

FIG. 3 is a graph showing the same GPC curve as in FIG. 2, in which ahigh-molecular-weight portion is hatched.

FIG. 4 is a graph showing one example of typical TMA measurement, theshutdown temperature is shown by an arrow.

DETAILED DESCRIPTION OF THE INVENTION [1] Composition and Structure ofthe Multi-Layer, Microporous Polyolefin Membrane

In an embodiment, the multi-layer, microporous membrane comprises twolayers. The first layer (e.g., the upper layer) comprises a firstmicroporous layer material, and the second layer (e.g., the bottomlayer) comprises a second microporous layer material. For example, themembrane has a planar top layer when viewed from above on an axisapproximately perpendicular to the transverse and machine directions ofthe membrane, with the bottom planar layer hidden from view by the toplayer. In another embodiment, the multi-layer, microporous polyolefinmembrane comprises three or more layers, wherein the outer layers (alsocalled the “surface” or “skin” layers) comprise the first microporouslayer material and at least one intermediate layer comprises the secondmicroporous layer material. In a related embodiment, where themulti-layer, microporous membrane comprises two layers, the first layerconsists essentially of (or consists of) the first microporous layermaterial and the second layer consists essentially of (or consists of)the second microporous layer material. In a related embodiment where themulti-layer, microporous membrane comprises three or more layers, theouter layers consist essentially of (or consists of) the firstmicroporous layer material and at least one intermediate layer consistsessentially of (or consists of) the second microporous layer material.The membrane can be referred to as a “polyolefin membrane” when themembrane contains polyolefin. While the membrane can contain polyolefinonly, this is not required, and it is within the scope of the inventionfor the membrane to contain polyolefin and materials that are notpolyolefin.

In yet another embodiment where the multi-layer, microporous membranecomprises three or more layers, the surface layers comprise (or consistessentially of, or consist of) the second microporous layer material andat least one intermediate layer comprises (or consists essentially of,or consists of) the first microporous layer material.

When the multi-layer, microporous membrane has three or more layers, themulti-layer, microporous polyolefin membrane has at least one layercomprising the first microporous layer material and at least one layercomprising the second microporous layer material.

In an embodiment, the sum of the thicknesses of the layers comprisingthe first layer material generally is in the range of about 3% to about90%, or about 10% to about 60% of the total thickness of the multi-layermicroporous membrane.

The first microporous layer material comprises a first polypropylene anda first polyethylene. The second microporous layer material comprises asecond polyethylene and a second polypropylene. The total amount ofpolyethylene in the multi-layer, microporous polyolefin membrane is inthe range of from about 9.5 wt. % to about 95 wt. %, based on the weightof the multi-layer, microporous polyolefin membrane. The total amount ofpolypropylene in the multi-layer, microporous polyolefin membrane is inthe range of from about 1.4 wt. % to about 90.5 wt. %, based on theweight of the multi-layer, microporous polyolefin membrane. The firstpolyethylene is present in the first microporous layer material in afirst polyethylene amount in the range of from about 50 wt. % to about99 wt. % based on the weight of the first microporous layer material.The first polypropylene is present in the first microporous layermaterial in a first polypropylene amount in the range of from about 1wt. % to about 50 wt. % based on the weight of the first microporouslayer material. The second polyethylene is present in the secondmicroporous layer material in a second polyethylene amount in the rangeof from about 5 wt. % to about 95 wt. % based on the weight of thesecond microporous layer material. The second polypropylene is presentin the second microporous layer material in a second polypropyleneamount in the range of from about 5 wt. % to about 95 wt. % based on theweight of the second microporous layer material.

The first and second polyethylene and the first and second polypropylenewill now be described in more detail.

A. The First Polyethylene

In an embodiment, the first polyethylene is a polyethylene having an Mwin the range of about 1×10⁴ to about 1×10⁷, or about 1×10⁵ to about5×10⁶, or about 2×10⁵ to about 3×10⁶. The first polyethylene can be oneor more varieties of polyethylene, e.g., PE1, PE2, etc. PE1 comprisespolyethylene having an Mw ranging from about 1×10⁴ to about 5×10⁵.Optionally, the PE1 can be one or more of an high density polyethylene(“HDPE”), a medium-density polyethylene, a branched low-densitypolyethylene, or a linear low-density polyethylene. Although it is notcritical, the Mw of high-density polyethylene can range, for example,from about 1×10⁵ to about 5×10⁵, or from about 2×10⁵ to about 4×10⁵. Inan embodiment, PE1 is at least one of (i) an ethylene homopolymer or(ii) a copolymer of ethylene and a third a-olefin such as propylene,butene-1, hexene-1, etc, typically in a relatively small amount comparedto the amount of ethylene. Such a copolymer can be produced using asingle-site catalyst.

In an embodiment, the first polyethylene comprises a secondpolyethylene, PE2. PE2 comprises polyethylene having an Mw of at leastabout 1×10⁶. For example, PE2 can be an ultra-high molecular weightpolyethylene (“UHMWPE”). In an embodiment, PE2 is at least one of (i) anethylene homopolymer or (ii) a copolymer of ethylene and a fourthα-olefin which is typically present in a relatively small amountcompared to the amount of ethylene. The fourth α-olefin can be, forexample, one or more of propylene, butene-1, pentene-1,hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methylmethacrylate, or styrene. Although it is not critical, the Mw of PE2 canrange from about 1×10⁶ to about 15×10⁶, or from about 1×10⁶ to about5×10⁶, or from about 1×10⁶ to about 3×10⁶.

In an embodiment, the first polyethylene comprises both PE1 and PE2. Inthis case, the amount of PE2 in the first polyethylene is in the rangeof about 0 wt % to about 50 wt %, or about 1 wt. % to about 50 wt. %,based on the weight of the first polyethylene.

In one embodiment, the first polyethylene has one or more of thefollowing independently-selected features:

-   (1) The first polyethylene comprises PE1.-   (2) The first polyethylene consists essentially of, or consists of,    PE1.-   (3) The PE1 is one or more of a high-density polyethylene, a    medium-density polyethylene, a branched low-density polyethylene, or    a linear low-density polyethylene.-   (4) PE1 is one or more of a high-density polyethylene having an Mw    ranging from about 1×10⁵ to about 5×10⁵, or from about 2×10⁵ to    about 4×10⁵.-   (5) PE1 is at least one of (i) an ethylene homopolymer or (ii) a    copolymer of ethylene and a third α-olefin selected from the group    of propylene, butene-1, hexene-1.-   (6) The first polyethylene comprises both PE1 and PE2.-   (7) PE2 has an Mw ranging from about 1×10⁶ to about 15×10⁶, or    optionally from about 1×10⁶ to about 5×10⁶, or optionally from about    1×10⁶ to about 3×10⁶.-   (8) PE2 is ultra-high-molecular-weight polyethylene.-   (9) PE2 is at least one of (i) an ethylene homopolymer or (ii) a    copolymer of ethylene and a fourth α-olefin selected from the group    of propylene, butene-1, hexene-1.-   (10) The first polyethylene has a molecular weight distribution    (“Mw/Mn”) of about 5 to about 300, or about 5 to about 100, or    optionally from about 5 to about 30.

B. The Second Polyethylene

The second polyethylene can comprise PE1, PE2, or both PE1 and PE2. Whenthe second polyethylene comprises PE1 and PE2, the amount of PE2 in thesecond polyethylene is in the range of about 0 wt % to about 50 wt %, orabout 1 wt. % to about 50 wt. % based on the weight of the secondpolyethylene.

C. The First Polypropylene

Besides polyethylene, the first and second microporous layer materialscomprise polypropylene. The polypropylene can be, for example, one ormore of (i) a propylene homopolymer or (ii) a copolymer of propylene anda fifth olefin. The copolymer can be a random or block copolymer. Thefifth olefin can be, e.g., one or more of α-olefins such as ethylene,butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1, vinylacetate, methyl methacrylate, and styrene, etc.; and diolefins such asbutadiene, 1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, etc. The amountof the fifth olefin in the copolymer is preferably in a range that doesnot adversely affect properties of the multi-layer microporous membranesuch as heat resistance, compression resistance, heat shrinkageresistance, etc. For example, the amount of the fifth olefin can be lessthan 10% by mol based on 100% by mol of the entire copolymer.Optionally, the polypropylene has one or more of the followingproperties: (i) the polypropylene has an Mw ranging from about 1×10⁴ toabout 4×10⁶, or about 3×10⁵ to about 3×10⁶; (ii) the polypropylene hasan Mw/Mn ranging from about 1.01 to about 100, or about 1.1 to about 50;(iii) the polypropylene's tacticity is isotactic; (iv) the polypropylenehas a heat of fusion of at least about 90 Joules/gram; (v) polypropylenehas a melting peak (second melt) of at least about 160° C., (vi) thepolypropylene has a Trouton's ratio of at least about 15 when measuredat a temperature of about 230° C. and a strain rate of 25 sec⁻¹; and/or(vii) the polypropylene has an elongational viscosity of at least about50,000 Pa sec at a temperature of 230° C. and a strain rate of 25 sec⁻¹.Optionally The polypropylene has an Mw/Mn ranging from about 1.01 toabout 100, or from about 1.1 to about 50.

D. The Second Polypropylene

Polypropylene preferably has a weight-average molecular weight of 6×10⁵or more, and a heat of fusion ΔHm (measured by a differential scanningcalorimeter (DSC) according to JIS K7122) of 90 J/g or more, a fractionof the polypropylene having a molecular weight of 1.8×10⁶ or more(determined from a molecular weight distribution) being 10% or more bymass. A temperature-elevating speed for the measurement of the heat offusion is preferably 3-20° C./minute, usually 10° C./minute. Becausepolypropylene having a weight-average molecular weight of less than6×10⁵ has low dispersibility in the polyethylene, its use makesstretching difficult, providing large micro-roughness to a surface ofthe second porous layer and large thickness variation to themulti-layer, microporous membrane. When a fraction of the polypropylenehaving a molecular weight of 1.8×10⁶ or more is less than 10% by mass ofthe polypropylene, the multi-layer, microporous membrane may haveundesirably low meltdown properties. When the polypropylene has a heatof fusion ΔHm of less than 90 J/g, the resultant multi-layer,microporous membrane may have low meltdown properties and permeability.

The weight-average molecular weight of the polypropylene is preferably6.5×10⁵ or more, more preferably 8×10⁵ or more. Though not particularlyrestricted, the Mw/Mn of the polypropylene is preferably 1-100. The heatof fusion ΔHm of the polypropylene may be 90 J/g, preferably 95 J/g ormore, more preferably 100 J/g or more. The molecular weightdistribution, Mw/Mn, is preferably 5 or less, more preferably 4 or less,most preferably 2.5 or less.

The polypropylene content may be 0.01-99.9% by mass, preferably 5-95% bymass, more preferably 20-80% by mass, most preferably 30-70% by mass ofthe entire polyolefin composition. When the polypropylene content isless than 0.01% by mass, the meltdown temperature may not increase to adesirable level. When the polypropylene content exceeds 99.9% by mass,the multi-layer, microporous membrane may have deteriorated thicknessuniformity and permeability.

As long as the above conditions of the weight-average molecular weight,a fraction having a molecular weight of 1.8×10⁶ or more (determined froma molecular weight distribution), and the heat of fusion are met, thetype of the polypropylene is not particularly restrictive, but may be apropylene homopolymer, a copolymer of propylene and the other α-olefin,or a mixture thereof, the homopolymer being preferable. The copolymermay be a random or block copolymer. In a propylene copolymer, thecomonomer may include, for example, α-olefins such as ethylene,1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, octene, vinylacetate, methyl methacrylate, and styrene. Optionally, the polypropylenehas one or more of the following properties: (i) the polypropylene hasan Mw ranging from about 1×10⁴ to about 4×10⁶, or about 6×10⁵ to about3×10⁶; (ii) the polypropylene has an Mw/Mn ranging from about 1.01 toabout 100, or about 1.1 to about 50; (iii) the polypropylene's tacticityis isotactic; (iv) the polypropylene has a heat of fusion of at leastabout 90 Joules/gram; (v) polypropylene has a melting peak (second melt)of at least about 160° C., (vi) the polypropylene has a Trouton's ratioof at least about 15 when measured at a temperature of about 230° C. anda strain rate of 25 sec⁻¹; and/or (vii) the polypropylene has anelongational viscosity of at least about 50,000 Pa sec at a temperatureof 230° C. and a strain rate of 25 sec⁻¹.

The Mw and Mn of polypropylene can be measured, e.g., by a GPC methodunder the following conditions. Measurement apparatus: Alliance 2000 GPCavailable from Waters Corp., Columns: Three PL Gel mixed-B availablefrom Polymer Laboratories. Column temperature: 145° C., Solvent (mobilephase): 1,2,4-trichlorobenzene, stabilized with 0.1 wt % BHT, 6 g/4 L.Solvent flow rate: 1.0 ml/minute. Sample concentration: 0.25 mg/mL(dissolved at 175° C. for 1 hour). Injected amount: 300 μl. Detector:Differential Refractometer available from Waters Corp. Calibrationcurve: Produced from a calibration curve of a set of single-dispersion,standard polystyrene sample using a predetermined conversion constant.

The heat of fusion ΔHm of polypropylene can be measured according to JISK7122 as follows: A PP sample was heat-treated at 190° C. for 10 minutesin a nitrogen atmosphere in a sample holder of a differential scanningcalorimeter (DSC-System 7 available from Perkin Elmer, Inc.), cooled to40° C. at a speed of 10° C./minute, kept at 40° C. for 2 minutes, andheated to 190° C. at a speed of 110° C./minute. As shown in FIG. 1, astraight line passing through points on a DSC curve (melting curve)obtained by the temperature-elevating process at 85° C. and 175° C. canbe drawn as a base line, and the amount of heat was calculated from anarea S1 of a hatched portion encircled by the base line and the DSCcurve. The amount of heat (unit: J) was divided by the weight (unit: g)of the sample to determine the heat of fusion ΔHm (unit: J/g).

The percentage (on a mass basis) of a polypropylene fraction having amolecular weight of 1.8×10⁶ or more can be determined as follows. Todetermine the amount of the entire polypropylene sample, an area S2 of ahatched portion encircled by the GPC curve and the base line in FIG. 2can be measured. To determine the amount of the fraction having amolecular weight of 1.8×10⁶ or more, an area S3 in FIG. 3 can bemeasured. The percentage of the fraction having a molecular weight of1.8×10⁶ or more was calculated by (S3/S2)×100 (mass %).

[2] Materials Used to Produce the Multi-Layer, Microporous PolyolefinMembrane

A. Polymer Resins used to Make the First Microporous Layer Material

In an embodiment, the first microporous layer material is made from afirst polyolefin solution. The first polyolefin solution comprises afirst polyolefin composition and a first process solvent. Since theprocess produces a multi-layer microporous membrane, the process solventis also referred to as a diluent or a membrane-forming solvent. Theresins used to make the first polyolefin composition will now bedescribed in more detail.

(1) The First Polyethylene Resin

In an embodiment, the first polyethylene resin comprises the firstpolyethylene, where the first polyethylene is as described above insection [1]. For example, the first polyethylene resin can be a mixtureof a polyethylene resin having a lower Mw than UHMWPE (such as HDPE) andUHMWPE resin.

The molecular weight distribution (Mw/Mn”) of the polyethylene in thefirst polyethylene resin is not critical. Mw/Mn is a measure of amolecular weight distribution; the larger this value, the wider themolecular weight distribution. Though not critical, the Mw/Mn of thepolyethylene in the first polyethylene resin can range from about 5 toabout 300, or from about 5 to about 100, or from about 5 to about 30.When the Mw/Mn is less than 5, it can be more difficult to extrude thefirst polyethylene resin. On the other hand, when the Mw/Mn is more than300, it can be more difficult to produce a relatively strong multi-layermicroporous membrane. Multi-stage polymerization can be used to obtainthe desired Mw/Mn ratio in the first polyethylene resin. For example, atwo-stage polymerization method can be used, forming a relativelyhigh-molecular-weight polymer component in the first stage, and forminga relatively low-molecular-weight polymer component in the second stage.While not required, this method can be used, for example, when the firstpolyethylene resin comprises PE1. When the first polyethylene resincomprises the PE1 and PE2, the desired Mw/Mn ratio of the polyethyleneresin can be selected by adjusting the relative molecular weights andrelative amounts of the first and second polyethylene.

(2) The First Polypropylene Resin

Besides the first polyethylene resin, the first polyolefin compositionfurther comprises a first polypropylene resin. In an embodiment, thefirst polypropylene resin comprises the first polypropylene, where thefirst polypropylene is as described above in section [1]. The firstpolypropylene resin can be, for example, one or more of (i) a propylenehomopolymer or (ii) a copolymer of propylene and a fifth olefin. Thecopolymer can be a random or block copolymer. The fifth olefin can be,e.g., one or more α-olefins such as ethylene, butene-1, pentene-1,hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methylmethacrylate, and styrene, etc.; and diolefins such as butadiene,1,5-hexadiene, 1,7-octadiene, 1,9-decadiene, etc. The amount of thefifth olefin in the copolymer should be in a range that does notadversely affect properties of the resulting multi-layer microporousmembrane such as heat resistance, compression resistance, heat shrinkageresistance, etc. For example, the amount of the fifth olefin can be lessthan 10% by mol based on 100% by mol of the entire copolymer.

While it is not critical, the Mw of the polypropylene in the firstpolypropylene resin can range from about 1×10⁴ to about 4×10⁶, or fromabout 3×10⁵ to about 3×10⁶. While it is not critical, the molecularweight distribution (Mw/Mn) of the polypropylene in the firstpolypropylene resin can range from about 1.01 to about 100, or fromabout 1.1 to about 50.

(3) Formulation

The amount of process solvent in the first polyolefin solution rangesfrom about 25 wt. % to about 99 wt. % based on the weight of the firstpolyolefin solution. In an embodiment, the amount of the firstpolyethylene resin in the first polyolefin composition ranges from about50 wt. % to about 99 wt. % based on the weight of the first polyolefincomposition. The balance of the first polyolefin composition can be thefirst polypropylene.

B. Polymer Resins used to Produce the Second Microporous Layer Material

In an embodiment, the second microporous layer material is made from asecond polyolefin solution that is generally selected independently ofthe first polyolefin solution. The second polyolefin solution comprisesa second polyolefin composition and a second process solvent which canbe the same as the first process solvent. As is the case in the firstpolyolefin solution, the second process solvent can be referred to as asecond membrane-forming solvent or a second diluent. In an embodiment,the second polyolefin composition comprises a second polyethylene resinand a second polypropylene resin. The second polyethylene resincomprises the second polyethylene resin as described above in section[1]. The second polypropylene resin comprises the second polypropyleneas described above in section [1].

The amount of process solvent in the second polyolefin solution rangesfrom about 25 wt. % to about 99 wt. % based on the weight of the secondpolyolefin solution. In an embodiment, the amount of the secondpolyethylene resin in the second polyolefin composition ranges fromabout 5 wt. % to about 95 wt. % based on the weight of the secondpolyolefin composition. The balance of the second polyolefin compositioncan be the second polypropylene.

C. Third Polyolefin

Although it is not required, each of the first and second polyolefincompositions can further comprise a third polyolefin selected from thegroup consisting of polybutene-1, polypentene-1, poly-4-methylpentene-1,polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl methacrylate,polystyrene and an ethylene·α-olefin copolymer (except for anethylene-propylene copolymer). In an embodiment where a third polyolefinis used, the third polyolefin can, for example, have an Mw in the rangeof about 1×10⁴ to about 4×10⁶. In addition to or besides the thirdpolyolefin, the first and/or second polyolefin composition can furthercomprise a polyethylene wax, e.g., one having an Mw in the range ofabout 1×10³ to about 1×10⁴. When used, these species should be presentin amounts less than an amount that would cause deterioration in thedesired properties (e.g., meltdown, shutdown, etc.) of the multi-layer,microporous membrane. When the third polyolefin is one or more ofpolybutene-1, polypentene-1, poly-4-methylpentene-1, polyhexene-1,polyoctene-1, polyvinyl acetate, polymethyl methacrylate, andpolystyrene, the third polyolefin need not be a homopolymer, but may bea copolymer containing other α-olefins.

The multi-layer microporous membrane generally comprises the polyolefinused to form the polyolefin solution. A small amount of washing solventand/or process solvent can also be present, generally in amounts lessthan 1 wt % based on the weight of the microporous polyolefin membrane.A small amount of polyolefin molecular weight degradation might occurduring processing, but this is acceptable. In an embodiment, molecularweight degradation during processing, if any, causes the value of Mw/Mnof the polyolefin in the membrane to differ from the Mw/Mn of the firstor second polyolefin solution by no more than about 50%, or no more thanabout 1%, or no more than about 0.1%.

[3] Production Method of Multi-Layer, Microporous Polyolefin Membrane

In an embodiment, the microporous polyolefin membrane is a two-layermembrane. In another embodiment, the microporous polyolefin membrane hasat least three layers. For the sake of brevity, the production of themicroporous polyolefin membrane will be mainly described in terms oftwo-layer and three-layer membranes, although those skilled in the artwill recognize that the same techniques can be applied to the productionof membranes or membranes having at least four layers.

In an embodiment, the three-layer microporous polyolefin membranecomprises first and third microporous layers constituting the outerlayers of the microporous polyolefin membrane and a second layersituated between (and optionally in planar contact with) the first andthird layers. In an embodiment, the first and third layers are producedfrom the first polyolefin solution and the second (or inner) layer isproduced from the second polyolefin solution. In another embodiment, thefirst and third layers are produced from the second polyolefin solutionand the second layer is produced from the first polyolefin solution.

A. First Production Method

The first method for producing a multi-layer membrane comprises thesteps of (1) combining (e.g., by melt-blending) a first polyolefincomposition and a membrane-forming solvent to prepare a first polyolefinsolution, (2) combining a second polyolefin composition and a secondmembrane-forming solvent to prepare a second polyolefin solution, (3)extruding (preferably simultaneously) the first and second polyolefinsolutions through at least one die to form an extrudate, (4) cooling theextrudate to form a cooled extrudate, e.g., a multi-layer, gel-likesheet, (5) removing the membrane-forming solvent from the multi-layer,sheet to form a solvent-removed sheet, and (6) drying thesolvent-removed gel-like sheet to remove volatile species, if any, inorder to form the multi-layer, microporous polyolefin membrane. Anoptional stretching step (7), and an optional hot solvent treatment step(8), etc. can be conducted between steps (4) and (5), if desired. Afterstep (6), an optional step (9) of stretching a multi-layer, microporousmembrane, an optional heat treatment step (10), an optionalcross-linking step with ionizing radiation (11), and an optionalhydrophilic treatment step (12), etc., can be conducted if desired. Theorder of the optional steps is not critical.

B. Preparation of First Polyolefin Solution

The first polyolefin composition comprises polyolefin resins asdescribed above that can be combined, e.g., by dry mixing or meltblending with an appropriate membrane-forming solvent to produce thefirst polyolefin solution. Optionally, the first polyolefin solution cancontain various additives such as one or more antioxidant, fine silicatepowder (pore-forming material), etc., provided these are used in aconcentration range that does not significantly degrade the desiredproperties of the multi-layer, microporous polyolefin membrane.

The first process solvent (i.e., the first membrane-forming solvent) ispreferably a solvent that is liquid at room temperature. While notwishing to be bound by any theory or model, it is believed that the useof a liquid solvent to form the first polyolefin solution makes itpossible to conduct stretching of the gel-like sheet at a relativelyhigh stretching magnification. In an embodiment, the firstmembrane-forming solvent can be at least one of aliphatic, alicyclic oraromatic hydrocarbons such as nonane, decane, decalin, p-xylene,undecane, dodecane, liquid paraffin, etc.; mineral oil distillateshaving boiling points comparable to those of the above hydrocarbons; andphthalates liquid at room temperature such as dibutyl phthalate, dioctylphthalate, etc. In an embodiment where it is desired to obtain amulti-layer, gel-like sheet having a stable liquid solvent content,non-volatile liquid solvents such as liquid paraffin can be used, eitheralone or in combination with other solvents. Optionally, a solvent whichis miscible with polyethylene in a melt blended state but solid at roomtemperature can be used, either alone or in combination with a liquidsolvent. Such solid solvent can include, e.g., stearyl alcohol, cerylalcohol, paraffin waxes, etc. Although it is not critical, it can bemore difficult to evenly stretch the gel-like sheet or resultingmembrane when the solution contains no liquid solvent.

The viscosity of the liquid solvent is not a critical parameter. Forexample, the viscosity of the liquid solvent can range from about 30 cStto about 500 cSt, or from about 30 cSt to about 200 cSt, at 25° C.Although it is not a critical parameter, when the viscosity at 25° C. isless than about 30 cSt, it can be more difficult to prevent foaming thepolyolefin solution, which can lead to difficulty in blending. On theother hand, when the viscosity is greater than about 500 cSt, it can bemore difficult to remove the liquid solvent from the multi-layermicroporous polyolefin membrane.

In an embodiment, the resins, etc., used to produce to the firstpolyolefin composition are dry mixed or melt-blended in, e.g., a doublescrew extruder or mixer. For example, a conventional extruder (or mixeror mixer-extruder) such as a double-screw extruder can be used tocombine the resins, etc., to form the first polyolefin composition. Themembrane-forming solvent can be added to the polyolefin composition (oralternatively to the resins used to produce the polyolefin composition)at any convenient point in the process. For example, in an embodimentwhere the first polyolefin composition and the first membrane-formingsolvent are melt-blended, the solvent can be added to the polyolefincomposition (or its components) at any of (i) before startingmelt-blending, (ii) during melt blending of the first polyolefincomposition, or (iii) after melt-blending, e.g., by supplying the firstmembrane-forming solvent to the melt-blended or partially melt-blendedpolyolefin composition in a second extruder or extruder zone locateddownstream of the extruder zone used to melt-blend the polyolefincomposition.

When melt-blending is used, the melt-blending temperature is notcritical. For example, the melt-blending temperature of the firstpolyolefin solution can range from about 10° C. higher than the meltingpoint Tm₁ of the first polyethylene resin to about 120° C. higher thanTm₁. For brevity, such a range can be represented as Tm₁+10° C. toTm₁+120° C. In an embodiment where the first polyethylene resin has amelting point of about 130° C. to about 140° C., the melt-blendingtemperature can be in the range of from about 140° C. to about 250° C.,or from about 170° C. to about 240° C.

When an extruder such as a double-screw extruder is used formelt-blending, the screw parameters are not critical. For example, thescrew can be characterized by a ratio L/D of the screw length L to thescrew diameter D in the double-screw extruder, which can range, forexample, from about 20 to about 100, or from about 35 to about 70.Although this parameter is not critical, when L/D is less than about 20,melt-blending can be more difficult, and when L/D is more than about100, faster extruder speeds might be needed to prevent excessiveresidence time of the polyolefin solution in the double-screw extruder(which can lead to undesirable molecular weight degradation). Althoughit is not a critical parameter, the cylinder (or bore) of thedouble-screw extruder can have an inner diameter of in the range ofabout 40 mm to about 100 mm, for example.

The amount of the first polyolefin composition in the first polyolefinsolution is not critical. In an embodiment, the amount of firstpolyolefin composition in the first polyolefin solution can range fromabout 1 wt. % to about 75 wt. %, based on the weight of the polyolefinsolution, for example from about 20 wt. % to about 70 wt. %. Althoughthe amount of first polyolefin composition in the first polyolefinsolution is not critical, when the amount is less than about 1 wt. %, itcan be more difficult to produce the multi-layer microporous polyolefinmembrane at an acceptably efficient rate. Moreover, when the amount isless than 1 wt. %, it can be more difficult to prevent swelling orneck-in at the die exit during extrusion, which can make it moredifficult to form and support the multi-layer, gel-like sheet, which isa precursor of the membrane formed during the manufacturing process. Onthe other hand, when the amount of first polyolefin composition solutionis greater than about 75 wt. %, it can be more difficult to form themulti-layer, gel-like sheet. The amount of the first polyethylene resinis preferably 1-50% by mass, more preferably 20-40% by mass, per 100% bymass of the first polyolefin solution. When the polyethylene resin isless than 1% by mass, large swelling or neck-in may occur at the dieexit during the extrusion of the first polyolefin solution to form agel-like molding, resulting in decrease in the formability andself-support of the gel-like molding. On the other hand, when thepolyethylene resin is more than 50% by mass, the formability of thegel-like molding may be deteriorated.

(2) Preparation of Second Polyolefin Solution

The second polyolefin solution can be prepared by the same methods usedto prepare the first polyolefin solution. For example, the secondpolyolefin solution can be prepared by melt-blending a second polyolefincomposition with a second membrane-forming solvent. The secondmembrane-forming solvent can be selected from among the same solvents asthe first membrane-forming solvent. And while the secondmembrane-forming solvent can be (and generally is) selectedindependently of the first membrane-forming solvent, the secondmembrane-forming solvent can be the same as the first membrane-formingsolvent, and can be used in the same relative concentration as the firstmembrane-forming solvent is used in the first polyolefin solution.

The second polyolefin composition is generally selected independently ofthe first polyolefin composition. The second polyolefin compositioncomprises the second polyethylene resin and the second polypropyleneresin.

In an embodiment, the method for preparing the second polyolefinsolution differs from the method for preparing the first polyolefinsolution, only in that the mixing temperature is preferably in a rangefrom the melting point (Tm2) of the second polypropylene to Tm2+90° C.,and that the polyolefin composition content is preferably 1-50% by mass,more preferably 20-40% by mass.

(3) Extrusion

In an embodiment, the first polyolefin solution is conducted from afirst extruder to a first die and the second polyolefin solution isconducted from a second extruder to a second die. A layered extrudate insheet form (i.e., a body significantly larger in the planar directionsthan in the thickness direction) can be extruded from the first andsecond die. Optionally, the first and second polyolefin solutions areco-extruded from the first and second die with a planar surface of afirst extrudate layer formed from the first polyolefin solution incontact with a planar surface of a second extrudate layer formed fromthe second polyolefin solution. A planar surface of the extrudate can bedefined by a first vector in the machine direction of the extrudate anda second vector in the transverse direction of the extrudate.

In an embodiment, a die assembly is used where the die assemblycomprises the first and second die, as for example when the first dieand the second die share a common partition between a region in the dieassembly containing the first polyolefin solution and a second region inthe die assembly containing the second polyolefin solution.

In another embodiment, a plurality of dies is used, with each dieconnected to an extruder for conducting either the first or secondpolyolefin solution to the die. For example, in one embodiment, thefirst extruder containing the first polyolefin solution is connected toa first die and a third die, and a second extruder containing the secondpolyolefin solution is connected to a second die. As is the case in thepreceding embodiment, the resulting layered extrudate can be co-extrudedfrom the first, second, and third die (e.g., simultaneously) to form athree-layer extrudate comprising a first and a third layer constitutingsurface layers (e.g., top and bottom layers) produced from the firstpolyolefin solution; and a second layer constituting a middle orintermediate layer of the extrudate situated between and in planarcontact with both surface layers, where the second layer is producedfrom the second polyolefin solution.

In yet another embodiment, the same die assembly is used but with thepolyolefin solutions reversed, i.e., the second extruder containing thesecond polyolefin solution is connected to the first die and the thirddie, and the first extruder containing the first polyolefin solution isconnected to the second die.

In any of the preceding embodiments, die extrusion can be conductedusing conventional die extrusion equipment. For example, extrusion canbe conducted by a flat die or an inflation die. In one embodiment usefulfor co-extrusion of multi-layer gel-like sheets, multi-manifoldextrusion can be used, in which the first and second polyolefinsolutions are conducted to separate manifolds in a multi-layer extrusiondie and laminated at a die lip inlet. In another such embodiment, blockextrusion can be used, in which the first and second polyolefinsolutions are first combined into a laminar flow (i.e., in advance),with the laminar flow then connected to a die. Because multi-manifoldand block processes are known to those skilled in the art of processingpolyolefin films (e.g., as disclosed in JP06-122142 A, JP06-106599A),they are deemed conventional, therefore, their operation will be notdescribed in detail.

Die selection is not critical, and, e.g., a conventionalmulti-layer-sheet-forming, flat or inflation die can be used. Die gap isnot critical. For example, the multi-layer-sheet-forming flat die canhave a die gap of about 0.1 mm to about 5 mm. Die temperature andextruding speed are also non-critical parameters. For example, the diecan be heated to a die temperature ranging from about 140° C. to about250° C. during extrusion. The extruding speed can range, for example,from about 0.2 m/minute to about 15 m/minute. The thickness of thelayers of the layered extrudate can be independently selected. Forexample, the gel-like sheet can have relatively thick surface layers (or“skin” layers) compared to the thickness of an intermediate layer of thelayered extrudate.

While the extrusion has been described in terms of embodiments producingtwo and three-layer extrudates, the extrusion step is not limitedthereto. For example, a plurality of dies and/or die assemblies can beused to produce multi-layer extrudates having four or more layers usingthe extrusion methods of the preceding embodiments. In such a layeredextrudate, each surface or intermediate layer can be produced usingeither the first polyolefin solution and/or the second polyolefinsolution.

(4) Formation of a multi-layer, gel-like sheet

The multi-layer extrudate can be formed into a multi-layer, gel-likesheet by cooling, for example. Cooling rate and cooling temperature arenot particularly critical. For example, the multi-layer, gel-like sheetcan be cooled at a cooling rate of at least about 50° C./minute untilthe temperature of the multi-layer, gel-like sheet (the coolingtemperature) is approximately equal to the multi-layer, gel-like sheet'sgelation temperature (or lower). In an embodiment, the extrudate iscooled to a temperature of about 25° C. or lower in order to form themulti-layer gel-like sheet. While not wishing to be bound by any theoryor model, it is believed that cooling the layered extrudate sets thepolyolefin micro-phases of the first and second polyolefin solutions forseparation by the membrane-forming solvent or solvents. It has beenobserved that in general a slower cooling rate (e.g., less than 50°C./minute) provides the multi-layer, gel-like sheet with largerpseudo-cell units, resulting in a coarser higher-order structure. On theother hand, a relatively faster cooling rate (e.g., 80° C./minute)results in denser cell units. Although it is not a critical parameter,when the cooling rate of the extrudate is less than 50° C./minute,increased polyolefin crystallinity in the layer can result, which canmake it more difficult to process the multi-layer, gel-like sheet insubsequent stretching steps. The choice of cooling method is notcritical. For example conventional sheet cooling methods can be used. Inan embodiment, the cooling method comprises contacting the layeredextrudate with a cooling medium such as cooling air, cooling water, etc.Alternatively, the extrudate can be cooled via contact with rollerscooled by a cooling medium, etc.

(5) Removal of the First and Second Membrane-Forming Solvents

In an embodiment, at least a portion of the first and secondmembrane-forming solvents are removed (or displaced) from themulti-layer gel-like sheet in order to form a solvent-removed gel-likesheet. A displacing (or “washing”) solvent can be used to remove (washaway, or displace) the first and second membrane-forming solvents. Whilenot wishing to be bound by any theory or model, it is believed thatbecause the polyolefin phases in the multi-layer gel-like sheet producedfrom the first polyolefin solution and the second polyolefin solution(i.e., the first polyolefin and the second polyolefin) are separatedfrom the membrane-forming solvent phase, the removal of themembrane-forming solvent provides a porous membrane constituted byfibrils forming a fine three-dimensional network structure and havingpores communicating three-dimensionally and irregularly. The choice ofwashing solvent is not critical provided it is capable of dissolving ordisplacing at least a portion of the first and/or secondmembrane-forming solvent. Suitable washing solvents include, forinstance, one or more of volatile solvents such as saturatedhydrocarbons such as pentane, hexane, heptane, etc.; chlorinatedhydrocarbons such as methylene chloride, carbon tetrachloride, etc.;ethers such as diethyl ether, dioxane, etc.; ketones such as methylethyl ketone, etc.; linear fluorocarbons such as trifluoroethane, C₆F₁₄,C₇F₁₆, etc.; cyclic hydrofluorocarbons such as C₅H₃F₇, etc.;hydrofluoroethers such as C₄F₉OCH₃, C₄F₉OC₂H₅, etc.; and perfluoroetherssuch as C₄F₉OCF₃, C₄F₉OC₂F₅, etc.

The method for removing the membrane-forming solvent is not critical,and any method capable of removing a significant amount of solvent canbe used, including conventional solvent-removal methods. For example,the multi-layer, gel-like sheet can be washed by immersing the sheet inthe washing solvent and/or showering the sheet with the washing solvent.The amount of washing solvent used is not critical, and will generallydepend on the method selected for removal of the membrane-formingsolvent. For example, the amount of washing solvent used can range fromabout 300 to about 30,000 parts by mass, based on the mass of thegel-like sheet. While the amount of membrane-forming solvent removed isnot particularly critical, generally a higher quality (more porous)membrane will result when at least a major amount of first and secondmembrane-forming solvent is removed from the gel-like sheet. In anembodiment, the membrane-forming solvent is removed from the gel-likesheet (e.g., by washing) until the amount of the remainingmembrane-forming solvent in the multi-layer gel-like sheet becomes lessthan 1 wt. %, based on the weight of the gel-like sheet.

(6) Drying of the Solvent-Removed Gel-Like Sheet

In an embodiment, the solvent-removed multi-layer, gel-like sheetobtained by removing at least a portion of the membrane-forming solventis dried in order to remove the washing solvent. Any method capable ofremoving the washing solvent can be used, including conventional methodssuch as heat-drying, wind-drying (moving air), etc. The temperature ofthe gel-like sheet during drying (i.e., drying temperature) is notcritical. For example, the drying temperature can be equal to or lowerthan the crystal dispersion temperature Tcd. Tcd is the lower of thecrystal dispersion temperature Tcd₁ of the first polyethylene resin andthe crystal dispersion temperature Tcd₂ of the second polyethylene resin(when used). For example, the drying temperature can be at least 5° C.below the crystal dispersion temperature Tcd. The crystal dispersiontemperature of the first and second polyethylene resin can be determinedby measuring the temperature characteristics of the kineticviscoelasticity of the polyethylene resin according to ASTM D 4065. Inan embodiment, at least one of the first or second polyethylene resinshave a crystal dispersion temperature in the range of about 90° C. toabout 100° C.

Although it is not critical, drying can be conducted until the amount ofremaining washing solvent is about 5 wt. % or less on a dry basis, i.e.,based on the weight of the dry multi-layer, microporous polyolefinmembrane. In another embodiment, drying is conducted until the amount ofremaining washing solvent is about 3 wt. % or less on a dry basis.Insufficient drying can be recognized because it generally leads to anundesirable decrease in the porosity of the multi-layer, microporousmembrane. If this is observed, an increased drying temperature and/ordrying time should be used. Removal of the washing solvent, e.g., bydrying or otherwise, results in the formation of the multi-layer,microporous polyolefin membrane.

(7) Stretching

Prior to the step for removing the membrane-forming solvents (namelyprior to step 5), the multi-layer, gel-like sheet can be stretched inorder to obtain a stretched, multi-layer, gel-like sheet. It is believedthat the presence of the first and second membrane-forming solvents inthe multi-layer, gel-like sheet results in a relatively uniformstretching magnification. Heating the multi-layer, gel-like sheet,especially at the start of stretching or in a relatively early stage ofstretching (e.g., before 50% of the stretching has been completed) isalso believed to aid the uniformity of stretching.

Neither the choice of stretching method nor the degree of stretchingmagnification are particularly critical. For example, any method capableof stretching the multi-layer, gel-like sheet to a predeterminedmagnification (including any optional heating) can be used. In anembodiment, the stretching can be accomplished by one or more oftenter-stretching, roller-stretching, or inflation stretching (e.g.,with air). Although the choice is not critical, the stretching can beconducted monoaxially (i.e., in either the machine or transversedirection) or biaxially (both the machine or transverse direction). Inan embodiment, biaxial stretching is used. In the case of biaxialstretching (also called biaxial orientation), the stretching can besimultaneous biaxial stretching, sequential stretching along one planaraxis and then the other (e.g., first in the transverse direction andthen in the machine direction), or multi-stage stretching (for instance,a combination of the simultaneous biaxial stretching and the sequentialstretching). In an embodiment, simultaneous biaxial stretching is used.

The stretching magnification is not critical. In an embodiment wheremonoaxial stretching is used, the linear stretching magnification canbe, e.g., about 2 fold or more, or about 3 to about 30 fold. In anembodiment where biaxial stretching is used, the linear stretchingmagnification can be, e.g., about 3 fold or more in any lateraldirection. In another embodiment, the area magnification resulting fromstretching is at least about 9 fold, or at least about 16 fold, or atleast about 25 fold. Although it is not a critical parameter, when thestretching results in an area magnification of at least about 9 fold,the multi-layer microporous polyolefin membrane has a relatively higherpin puncture strength. When attempting an area magnification of morethan about 400 fold, it can be more difficult to operate the stretchingapparatus.

The temperature of the multi-layer, gel-like sheet during stretching(namely the stretching temperature) is not critical. In an embodiment,the temperature of the gel-like sheet during stretching can be about(Tm+10° C.) or lower, or optionally in a range that is higher than Tcdbut lower than Tm, wherein Tm is the lesser of the melting point Tm₁ ofthe first polyethylene and the melting point Tm₂ of the secondpolyethylene (when used). Although this parameter is not critical, whenthe stretching temperature is higher than approximately the meltingpoint Tm+10° C., at least one of the first or second polyethylene can bein the molten state, which can make it more difficult to orient themolecular chains of the polyolefin in the multi-layer gel-like sheetduring stretching. And when the stretching temperature is lower thanapproximately Tcd, at least one of the first or second polyethylene canbe so insufficiently softened that it is difficult to stretch themulti-layer, gel-like sheet without breakage or tears, which can resultin a failure to achieve the desired stretching magnification. In anembodiment, the stretching temperature ranges from about 90° C. to about140° C., or from about 100° C. to about 130° C.

While not wishing to be bound by any theory or model, it is believedthat such stretching causes cleavage between polyethylene lamellas,making the polyethylene phases finer and forming large numbers offibrils. The fibrils form a three-dimensional network structure(three-dimensionally irregularly connected network structure).Consequently, the stretching when used generally makes it easier toproduce a relatively high-mechanical strength multi-layer, microporouspolyolefin membrane with a relatively large pore size. Such multi-layer,microporous membranes are believed to be particularly suitable for useas battery separators.

Optionally, stretching can be conducted in the presence of a temperaturegradient in a thickness direction (i.e., a direction approximatelyperpendicular to the planar surface of the multi-layer, microporouspolyolefin membrane). In this case, it can be easier to produce amulti-layer, microporous polyolefin membrane with improved mechanicalstrength. The details of this method are described in Japanese Patent3347854.

(8) Hot Solvent Treatment Step

Although it is not required, the multi-layer, gel-like sheet can betreated with a hot solvent between steps (4) and (5). When used, it isbelieved that the hot solvent treatment provides the fibrils (such asthose formed by stretching the multi-layer gel-like sheet) with arelatively thick leaf-vein-like structure. Such a structure, it isbelieved, makes it less difficult to produce a multi-layer, microporousmembrane having large pores with relatively high strength andpermeability. The term “leaf-vein-like” means that the fibrils havethick trunks and thin fibers extending therefrom in a network structure.The details of this method are described in WO 2000/20493.

(9) Stretching of Multi-Layer, Microporous Membrane (“Dry Stretching”)

In an embodiment, the dried multi-layer, microporous membrane of step(6) can be stretched, at least monoaxially. The stretching methodselected is not critical, and conventional stretching methods can beused such as by a tenter method, etc. While it is not critical, themembrane can be heated during stretching. While the choice is notcritical, the stretching can be monoaxial or biaxial. When biaxialstretching is used, the stretching can be conducted simultaneously inboth axial directions, or, alternatively, the multi-layer, microporouspolyolefin membrane can be stretched sequentially, e.g., first in themachine direction and then in the transverse direction. In anembodiment, simultaneous biaxial stretching is used. When themulti-layer gel-like sheet has been stretched as described in step (7)the stretching of the dry multi-layer, microporous polyolefin membranein step (9) can be called dry-stretching, re-stretching, ordry-orientation.

The temperature of the dry multi-layer, microporous membrane duringstretching (the “dry stretching temperature”) is not critical. In anembodiment, the dry stretching temperature is approximately equal to themelting point Tm or lower, for example in the range of from about thecrystal dispersion temperature Tcd to the about the melting point Tm.When the dry stretching temperature is higher than Tm, it can be moredifficult to produce a multi-layer, microporous polyolefin membranehaving a relatively high compression resistance with relatively uniformair permeability characteristics, particularly in the transversedirection when the dry multi-layer, microporous polyolefin membrane isstretched transversely. When the stretching temperature is lower thanTcd, it can be more difficult to sufficiently soften the first andsecond polyolefins, which can lead to tearing during stretching, and alack of uniform stretching. In an embodiment, the dry stretchingtemperature ranges from about 90° C. to about 135° C., or from about 95°C. to about 130° C.

When dry-stretching is used, the stretching magnification is notcritical. For example, the stretching magnification of the multi-layer,microporous membrane can range from about 1.1 fold to about 1.8 fold inat least one lateral (planar) direction. Thus, in the case of monoaxialstretching, the stretching magnification can range from about 1.1 foldto about 1.8 fold in the longitudinal direction (i.e., the “machinedirection”) or the transverse direction, depending on whether themembrane is stretched longitudinally or transversely. Monoaxialstretching can also be accomplished along a planar axis between thelongitudinal and transverse directions.

In an embodiment, biaxial stretching is used (i.e., stretching along twoplanar axis) with a stretching magnification of about 1.1 fold to about1.8 fold along both stretching axes, e.g., along both the longitudinaland transverse directions. The stretching magnification in thelongitudinal direction need not be the same as the stretchingmagnification in the transverse direction. In other words, in biaxialstretching, the stretching magnifications can be selected independently.In an embodiment, the dry-stretching magnification is the same in bothstretching directions.

(10) Heat Treatment

In an embodiment, the dried multi-layer, microporous membrane can beheat-treated following step (6). It is believed that heat-treatingstabilizes the polyolefin crystals in the dried multi-layer, microporouspolyolefin membrane to form uniform lamellas. In an embodiment, the heattreatment comprises heat-setting and/or annealing. When heat-setting isused, it can be conducted using conventional methods such as tentermethods and/or roller methods. Although it is not critical, thetemperature of the dried multi-layer, microporous polyolefin membraneduring heat-setting (i.e., the “heat-setting temperature”) can rangefrom the Tcd to about the Tm. In an embodiment, the heat-settingtemperature ranges from about the dry stretching temperature of themulti-layer, microporous polyolefin membrane ±5° C., or about the drystretching temperature of the multi-layer, microporous polyolefinmembrane ±3° C.

Annealing differs from heat-setting in that it is a heat treatment withno load applied to the multi-layer, microporous polyolefin membrane. Thechoice of annealing method is not critical, and it can be conducted, forexample, by using a heating chamber with a belt conveyer or anair-floating-type heating chamber. Alternatively, the annealing can beconducted after the heat-setting with the tenter clips slackened. Thetemperature of the multi-layer, microporous polyolefin membrane duringannealing (i.e., the annealing temperature) is not critical. In anembodiment, the annealing temperature ranges from about the meltingpoint Tm or lower, or in a range from about 60° C. to (Tm−10° C.). It isbelieved that annealing makes it less difficult to produce amulti-layer, microporous polyolefin membrane having relatively highpermeability and strength.

(11) Cross-Linking

In an embodiment, the multi-layer, microporous polyolefin membrane canbe cross-linked (e.g., by ionizing radiation rays such as α-rays,β-rays, γ-rays, electron beams, etc.) after step (6). For example, whenirradiating electron beams are used for cross-linking, the amount ofelectron beam radiation can be about 0.1 Mrad to about 100 Mrad, usingan accelerating voltage in the range of about 100 kV to about 300 kV. Itis believed that the cross-linking treatment makes it less difficult toproduce a multi-layer, microporous polyolefin membrane with relativelyhigh meltdown temperature.

(12) Hydrophilizing Treatment

In an embodiment, the multi-layer, microporous polyolefin membrane canbe subjected to a hydrophilic treatment (i.e., a treatment which makesthe multi-layer, microporous polyolefin membrane more hydrophilic). Thehydrophilic treatment can be, for example, a monomer-grafting treatment,a surfactant treatment, a corona-discharging treatment, etc. In anembodiment, the monomer-grafting treatment is used after thecross-linking treatment.

When a surfactant treatment is used, any of nonionic surfactants,cationic surfactants, anionic surfactants and amphoteric surfactants canbe used, for example, either alone or in combination. In an embodiment,a nonionic surfactant is used. The choice of surfactant is not critical.For example, the multi-layer, microporous polyolefin membrane can bedipped in a solution of the surfactant and water or a lower alcohol suchas methanol, ethanol, isopropyl alcohol, etc., or coated with thesolution, e.g., by a doctor blade method.

B. Second Production Method

The second method for producing the multi-layer, microporous polyolefinmembrane comprises the steps of (1) combining (e.g., by melt-blending) afirst polyolefin composition and a first membrane-forming solvent toprepare a first polyolefin solution, (2) combining a second polyolefincomposition and a second membrane-forming solvent to prepare a secondpolyolefin solution, (3) extruding the first polyolefin solution througha first die and the second solution through a second die and thenlaminating the extruded first and second polyolefin solutions to form amulti-layer extrudate, (4) cooling the multi-layer extrudate to form amulti-layer, gel-like sheet, (5) removing at least a portion of themembrane-forming solvent from the multi-layer, gel-like sheet to form asolvent-removed gel-like sheet, and (6) drying the solvent-removedgel-like sheet in order to form the multi-layer, microporous membrane.An optional stretching step (7), and an optional hot solvent treatmentstep (8), etc., can be conducted between steps (4) and (5), if desired.After step (6), an optional step (9) of stretching a multi-layer,microporous membrane, an optional heat treatment step (10), an optionalcross-linking step with ionizing radiations (11), and an optionalhydrophilic treatment step (12), etc., can be conducted.

The process steps and conditions of the second production method aregenerally the same as those of the analogous steps described inconnection with the first production method, except for step (3).Consequently, step (3) will be explained in more detail.

The type of die used is not critical provided the die is capable offorming an extrudate that can be laminated. In one embodiment, sheetdies (which can be adjacent or connected) are used to form theextrudates. The first and second sheet dies are connected to first andsecond extruders, respectively, where the first extruder contains thefirst polyolefin solution and the second extruder contains the secondpolyolefin solution. While not critical, lamination is generally easierto accomplish when the extruded first and second polyolefin solution arestill at approximately the extrusion temperature. The other conditionsmay be the same as in the first method.

In another embodiment, the first, second, and third sheet dies areconnected to first, second and third extruders, where the first andthird sheet dies contain the first polyolefin solutions, and the secondsheet die contains the second polyolefin solution. In this embodiment, alaminated extrudate is formed constituting outer layers comprising theextruded first polyolefin solution and one intermediate comprising theextruded second polyolefin solution.

In yet another embodiment, the first, second, and third sheet dies areconnected to first, second, and third extruders, where the second sheetdie contains the first polyolefin solution, and the first and thirdsheet dies contain the second polyolefin solution. In this embodiment, alaminated extrudate is formed constituting outer layers comprising theextruded second polyolefin solution and one intermediate comprisingextruded first polyolefin solution.

C. Third Production Method

The third method for producing the multi-layer, microporous polyolefinmembrane comprises the steps of (1) combining (e.g., by melt-blending) afirst polyolefin composition and a membrane-forming solvent to prepare afirst polyolefin solution, (2) combining a second polyolefin compositionand a second membrane-forming solvent to prepare a second polyolefinsolution, (3) extruding the first polyolefin solution through at leastone first die to form at least one first extrudate, (4) extruding thesecond polyolefin solution through at least one second die to form atleast one second extrudate, (5) cooling first and second extrudates toform at least one first gel-like sheet and at least one second gel-likesheet, (6) laminating the first and second gel-like sheet to form amulti-layer, gel-like sheet, (7) removing at least a portion of themembrane-forming solvent from the resultant multi-layer, gel-like sheetto form a solvent-removed gel-like sheet, and (8) drying thesolvent-removed gel-like sheet in order to form the multi-layer,microporous membrane. An optional stretching step (9), and an optionalhot solvent treatment step (10), etc., can be conducted between steps(5) and (6) or between steps (6) and (7), if desired. After step (8), anoptional step (11) of stretching a multi-layer, microporous membrane, anoptional heat treatment step (12), an optional cross-linking step withionizing radiations (13), and an optional hydrophilic treatment step(14), etc., can be conducted.

The main difference between the third production method and the secondproduction method is in the order of the steps for laminating andcooling.

In the second production method, laminating the first and secondpolyolefin solutions is conducted before the cooling step. In the thirdproduction method, the first and second polyolefin solutions are cooledbefore the laminating step.

The steps of (1), (2), (7) and (8) in the third production method can bethe same as the steps of (1), (2), (5) and (6) in the first productionmethod as described above. For the extrusion of the first polyolefinsolution through the first die, the conditions of step (3) of the secondproduction method can be used for step (3) of the third productionmethod. For the extrusion of the second solution through the second die,the conditions of step (4) in the third production method can be thesame as the conditions of step (3) in the second production method. Inone embodiment, either the first or second polyolefin solution isextruded through a third die. In this way, a multi-layer laminate can beformed having two layers produced from the first polyolefin solution anda single layer produced from the second polyolefin solution, or viceversa.

Step (5) of the third production method can be the same as step (4) inthe first production method except that in the third production methodthe first and second gel-like sheets are formed separately.

The step (6) of laminating the first and second gel-like sheets will nowbe explained in more detail. The choice of lamination method is notparticularly critical, and conventional lamination methods such asheat-induced lamination can be used to laminate the multi-layer gel-likesheet. Other suitable lamination methods include, for example,heat-sealing, impulse-sealing, ultrasonic-bonding, etc., either alone orin combination. Heat-sealing can be conducted using, e.g., one or morepair of heated rollers where the gel-like sheets are conducted throughat least one pair of the heated rollers. Although the heat-sealingtemperature and pressure are not particularly critical, sufficientheating and pressure should be applied for a sufficient time to ensurethat the gel-like sheets are appropriately bonded to provide amulti-layer, microporous membrane with relatively uniform properties andlittle tendency toward delamination. In an embodiment, the heat-sealingtemperature can be, for instance, about 90° C. to about 135° C., or fromabout 90° C. to about 115° C. In an embodiment, the heat-sealingpressure can be from about 00.1 MPa to about −50 MPa.

As is the case in the first and second production method, the thicknessof the layers formed from the first and second polyolefin solution(i.e., the layers comprising the first and second microporous layermaterials) can be controlled by adjusting the thickness of the first andsecond gel-like sheets and by the amount of stretching (stretchingmagnification and dry stretching magnification), when one or morestretching steps are used. Optionally, the lamination step can becombined with a stretching step by passing the gel-like sheets throughmulti-stages of heated rollers.

In an embodiment, the third production method forms a multi-layer,polyolefin gel-like sheet having at least three layers. For example,after cooling two extruded first polyolefin solutions and one extrudedsecond polyolefin solution to form the gel-like sheets, the multi-layergel-like sheet can be laminated with outer layers comprising theextruded first polyolefin solution and an intermediate layer comprisingthe extruded second polyolefin solution. In another embodiment, aftercooling two extruded second polyolefin solutions and one extruded firstpolyolefin solution to form the gel-like sheets, the multi-layergel-like sheet can be laminated with outer layers comprising theextruded second polyolefin solution and an intermediate layer comprisingthe extruded first polyolefin solution.

The stretching step (9) and the hot solvent treatment step (10) can bethe same as the stretching step (7) and the hot solvent treatment step(8) as described for the first production method, except stretching step(9) and hot solvent treatment step (10) are conducted on the firstand/or second gel-like sheets. The stretching temperatures of the firstand second gel-like sheets are not critical. For example, the stretchingtemperatures of the first gel-like sheet can be, e.g., Tm₁+10° C. orlower, or optionally about Tcd₁ or higher but lower than about Tm₁. Thestretching temperature of the second gel-like sheet can be, e.g.,Tm₂+10° C. or lower, or optionally about Tcd₂ or higher but lower thanabout Tm₂.

D. Fourth Production Method

The fourth method for producing the multi-layer, microporous polyolefinmembrane comprises the steps of (1) combining (e.g., by melt-blending) afirst polyolefin composition and a membrane-forming solvent to prepare afirst polyolefin solution, (2) combining a second polyolefin compositionand a second membrane-forming solvent to prepare a second polyolefinsolution, (3) extruding the first polyolefin solution through at leastone first die to form at least one first extrudate, (4) extruding thesecond polyolefin solution through at least one second die to form atleast one second extrudate, (5) cooling first and second extrudates toform at least one first gel-like sheet and at least one second gel-likesheet, (6) removing at least a portion of the first and secondmembrane-forming solvents from the first and second gel-like sheets toform solvent-removed first and second gel-like sheets, (7) drying thesolvent-removed first and second gel-like sheets to form at least onefirst polyolefin membrane and at least one second polyolefin membrane,and (8) laminating the first and second microporous polyolefin membranesin order to form the multi-layer, microporous polyolefin membrane.

A stretching step (9), a hot solvent treatment step (10), etc., can beconducted between steps (5) and (6), if desired. A stretching step (11),a heat treatment step (12), etc., can be conducted between steps (7) and(8), if desired. After step (8), a step (13) of stretching amulti-layer, microporous membrane, a heat treatment step (14), across-linking step with ionizing radiations (15), a hydrophilictreatment step (16), etc., can be conducted if desired.

Steps (1) and (2) in the fourth production method can be conducted underthe same conditions as steps of (1) and (2) in the first productionmethod. Steps (3), (4), and (5) in the fourth production method can beconducted under the same conditions as steps (3), (4), and (5) in thethird method. Step (6) in the fourth production method can be conductedunder the same conditions as step (5) in the first production methodexcept for removing the membrane-forming solvent from the first andsecond gel-like sheets. Step (7) in the fourth production method can beconducted under the same conditions as step (6) in the first productionmethod except that in the fourth production method the first and secondsolvent-removed gel-like sheets are dried separately. Step (8) in thefourth production method can be conducted under the same conditions asthe step (6) in the third production method except for laminating thefirst and second polyolefin microporous membranes. The stretching step(9) and the hot solvent treatment step (10) in the fourth productionmethod can be conducted under the same conditions as step (9) and (10)in the third production method. The stretching step (11) and the heattreatment step (12) in the fourth production method can be conductedunder the same conditions as steps (9) and (10) in the first productionmethod except that in the fourth production method the first and secondpolyolefin microporous membranes are stretched and/or heat treated.

In an embodiment, in the stretching step (11) in the fourth productionmethod, the stretching temperature of the first polyolefin microporousmembranes can be about Tm₁ or lower, or optionally about Tcd₁ to aboutTm₁, and the stretching temperature of the second polyolefin microporousmembrane can be about Tm₂ or lower, or optionally about Tcd₂ to aboutTm₂.

In an embodiment, the heat treatment step (12) in the fourth productionmethod can be HS and/or annealing. For example, in the heat treatmentstep (12) in the fourth production method, the heat-setting temperatureof the first polyolefin microporous membranes can be about Tcd₁ to aboutTm₁, or optionally about the dry stretching temperature ±5° C., oroptionally about the dry stretching temperature ±3° C. In an embodiment,in the heat treatment step (12) in the fourth production method, theheat-setting temperature of the second microporous membrane can be aboutTcd₂ to about Tm₂, or optionally the dry stretching temperature ±5° C.,or optionally the dry stretching temperature ±3° C. When the HS is used,it can be conducted by, e.g., a tenter method or a roller method.

In an embodiment, in the heat treatment step (12) in the fourthproduction method, the annealing temperature of the first microporousmembrane can be about Tm₁ or lower, or optionally about 60° C. to about(Tm₁−10° C.). In an embodiment, in the heat treatment step (12) in thefourth production method, the annealing temperature of the secondmicroporous membranes can be about Tm₂ or lower, or optionally about 60°C. to about (Tm₂−10° C.).

The conditions in step (13), stretching a multi-layer, microporousmembrane, a heat treatment step (14), a cross-linking step with ionizingradiations (15), and a hydrophilic treatment step (16) in the fourthproduction method can be the same as those for steps (9), (10), (11) and(12) in the first production method.

[4] The Properties of a Multi-Layer, Microporous Polyolefin Membrane

In an embodiment, the multi-layer, microporous polyolefin membrane has athickness ranging from about 3 μm to about 200 μm, or about 5 μm toabout 50 μm. Optionally, the multi-layer, microporous polyolefinmembrane has one or more of the following characteristics.

A. Porosity of about 25% to about 80%

When the porosity is less than 25%, the multi-layer, microporouspolyolefin membrane generally does not exhibit the desired airpermeability for use as a battery separator. When the porosity exceeds80%, it is more difficult to produce a battery separator of the desiredstrength, which can increase the likelihood of internal electrodeshort-circuiting.

B1. Air Permeability of about 20 seconds/100 cm³ to about 700seconds/100 cm³ (Converted to Value at 20-μm Thickness)

When the air permeability of the multi-layer, microporous polyolefinmembrane (as measured according to JIS P8117) ranges from about 20seconds/100 cm³ to about 700 seconds/100 cm³, it is less difficult toform batteries having the desired charge storage capacity and desiredcyclability. When the air permeability is less than about 20 seconds/100cm³, it is more difficult to produce a battery having the desiredshutdown characteristics, particularly when the temperatures inside thebatteries are elevated. Air permeability P₁ measured on a multi-layer,microporous membrane having a thickness T₁ according to JIS P8117 can beconverted to air permeability P₂ at a thickness of 20 μm by the equationof P₂=(P₁×20)/T₁.

B2. Air Permeability after Heat Compression of about 100 seconds/100 cm³to about 1000 seconds/100 cm³

The present multi-layer microporous membrane when heat-compressed at 90°C. under pressure of 2.2 MPa for 5 minutes has air permeability (asmeasured according to JIS P8117) of about 1000 sec/100 cm³ or less, suchas from about 100 to about 1000 sec/100 cm³. Batteries using suchmembranes have suitably large capacity and cyclability. The airpermeability after heat compression is preferably, for example, 950sec/100 cm³ or less.

C. Pin Puncture Strength of about 2,000 mN/20 μm or more

The pin puncture strength (converted to the value at a 20-μm membranethickness) is the maximum load measured when the multi-layer,microporous polyolefin membrane is pricked with a needle 1 mm indiameter with a spherical end surface (radius R of curvature: 0.5 mm) ata speed of 2 mm/second. When the pin puncture strength of themulti-layer, microporous polyolefin membrane is less than 2,000 mN/20μm, it is more difficult to produce a battery having the desiredmechanical integrity, durability, and toughness.

H. Shutdown Temperature of about 140° C. or lower

When the shutdown temperature of the multi-layer, microporous polyolefinmembrane exceeds 140° C., it is more difficult to produce a batteryseparator with the desired shutdown response when the battery isoverheated. One way to determine shutdown temperature involvesdetermining the temperature at a point of inflection observed near themelting point of the multi-layer, microporous membrane, under thecondition that a test piece of 3 mm in the longitudinal direction and 10mm in the transverse direction is heated from room temperature at aspeed of 5° C./minute while drawing the test piece in the longitudinaldirection under a load of 2 g. In an embodiment, the shutdowntemperature is in the range of about 120-140° C. The measurement can bemade as follows. Using a thermomechanical analyzer (TMA/SS6000 availablefrom Seiko Instruments Inc.), a test piece of the multi-layer,microporous membrane measuring 10 mm (TD)×3 mm (MD) can be heated fromroom temperature at a speed of 5° C./min, while pulling the test piecein a longitudinal direction at a constant load of 2 gf, and thetemperature at an inflection point of the sample length observed nearthe melting point of the test piece can be defined as the “shutdowntemperature.” (see FIG. 4, for example).

I. Meltdown Temperature of at least about 170° C.

In an embodiment, the meltdown temperature can range from about 170° C.to about 190° C. One way to measure meltdown temperature involvesdetermining the temperature at which a multi-layer, microporouspolyolefin membrane test piece of 3 mm in the longitudinal direction and10 mm in the transverse direction is broken by melting, under theconditions that the test piece is heated from room temperature at aheating rate of 5° C./minute while drawing the test piece in thelongitudinal direction under a load of 2 g.

K. Battery Capacity Recovery Ratio of 70% or more (Retention Property ofLithium Secondary Battery)

When the lithium ion secondary battery comprising a separator formed bya multi-layer, microporous membrane is stored at a temperature of 80° C.for 30 days, it is desired that the battery capacity recovery ratio[(capacity after high-temperature storing)/(initial capacity)]×100(%)should be 70% or more. The battery capacity recovery ratio is preferably75% or more.

L. Thickness Variation Ratio of 20% or less after Heat Compression

The thickness variation ratio after heat compression at 90° C. underpressure of 2.2 MPa for 5 minutes is generally 20% or less per 100% ofthe thickness before compression, preferably less than 10%. Batteriescomprising microporous membrane separators with a thickness variationratio of 20% or less have suitably large capacity and good cyclability.

[5] Battery Separator

In and embodiment, the battery separator formed by the abovemulti-layer, microporous polyolefin membrane has a thickness in therange of about 3 μm to about 200 μm, or about 5 μm to about 50 μm.Depending, e.g., on the choice of electrolyte, separator swelling mightincrease the final thickness to a value larger than 200 μm.

[6] Battery

In an embodiment, the multi-layer, microporous polyolefin membrane canbe used as a separator for primary and secondary batteries such aslithium ion batteries, lithium-polymer secondary batteries,nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries,nickel-zinc secondary batteries, silver-zinc secondary batteries, andparticularly for lithium ion secondary batteries. Explanations will bemade below on the lithium ion secondary batteries.

The lithium secondary battery comprises a cathode, an anode, and aseparator located between the anode and the cathode. The separatorgenerally contains an electrolytic solution (electrolyte). The electrodestructure is not critical, and conventional electrode structures can beused. The electrode structure may be, for instance, a coin type in whicha disc-shaped cathode and anode are opposing, a laminate type in which aplanar cathode and anode are alternately laminated with at least oneseparator situated between the anode and the cathode, a toroidal type inwhich ribbon-shaped cathode and anode are wound, etc.

The cathode generally comprises a current collector, and acathodic-active material layer capable of absorbing and discharginglithium ions, which is formed on the current collector. Thecathodic-active materials can be, e.g., inorganic compounds such astransition metal oxides, composite oxides of lithium and transitionmetals (lithium composite oxides), transition metal sulfides, etc. Thetransition metals can be, e.g., V, Mn, Fe, Co, Ni, etc. In anembodiment, the lithium composite oxides are lithium nickelate, lithiumcobaltate, lithium manganate, laminar lithium composite oxides based onα-NaFeO₂, etc. The anode generally comprises a current collector, and anegative-electrode active material layer formed on the currentcollector. The negative-electrode active materials can be, e.g.,carbonaceous materials such as natural graphite, artificial graphite,cokes, carbon black, etc.

The electrolytic solutions can be obtained by dissolving lithium saltsin organic solvents. The choice of solvent and/or lithium salt is notcritical, and conventional solvents and salts can be used. The lithiumsalts can be, e.g., LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃,LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, LiN(C₂F₅SO₂)₂, LiPF₄(CF₃)₂,LiPF₃(C₂F₅)₃, lower aliphatic carboxylates of lithium, LiAlCl₄, etc. Thelithium salts may be used alone or in combination. The organic solventscan be organic solvents having relatively high boiling points (comparedto the battery's shut-down temperature) and high dielectric constants.Suitable organic solvents include ethylene carbonate, propylenecarbonate, ethylmethyl carbonate, γ-butyrolactone, etc.; organicsolvents having low boiling points and low viscosity such astetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane,dimethyl carbonate, diethyl carbonate, and the like, including mixturesthereof. Because the organic solvents generally having high dielectricconstants generally also have a high viscosity, and vice versa, mixturesof high- and low-viscosity solvents can be used.

When the battery is assembled, the separator is generally impregnatedwith the electrolytic solution, so that the separator (multi-layer,microporous membrane) is provided with ion permeability. The choice ofimpregnation method is not critical, and conventional impregnationmethods can be used. For example, the impregnation treatment can beconducted by immersing the multi-layer, microporous membrane in anelectrolytic solution at room temperature.

The method selected for assembling the battery is not critical, andconventional battery-assembly methods can be used. For example, when acylindrical battery is assembled, a cathode sheet, a separator formed bythe multi-layer, microporous membrane and an anode sheet are laminatedin this order, and the resultant laminate is wound to a toroidal-typeelectrode assembly. A second separator might be needed to preventshort-circuiting of the toroidal windings. The resultant electrodeassembly can be deposited into a battery can and then impregnated withthe above electrolytic solution, and a battery lid acting as a cathodeterminal provided with a safety valve can be caulked to the battery canvia a gasket to produce a battery.

[7] Examples

The present invention will be explained in more detail referring to thefollowing non-limiting examples.

Example 1

(1) Preparation of First Polyolefin Solution

A first polyolefin composition comprising (a) 82% of PE1 having a weightaverage molecular weight of 3.0×10⁵ and a molecular weight distributionof 8.6, (b) 8% of PE2 having a weight average molecular weight of2.0×10⁶ and a molecular weight distribution of 8, (c) 10% of firstpolypropylene resin having a weight average molecular weight of1.40×10⁶, a heat of fusion of 111.6 J/g, a fraction having a molecularweight of 1.8×10⁶ or more with the fraction being 25.3% and a molecularweight distribution of 2.6, percentages by weight of the firstpolyolefin composition, is prepared by dry-blending. The polyethyleneresin in the composition has a melting point of 135° C. and a crystaldispersion temperature of 100° C.

Twenty-five parts by weight of the resultant first polyolefincomposition is charged into a strong-blending double-screw extruderhaving an inner diameter of 58 mm and L/D of 42, and 75 parts by mass ofliquid paraffin (50 cst at 40° C.) is supplied to the double-screwextruder via a side feeder. Melt-blending is conducted at 210° C. and200 rpm to prepare a first polyolefin solution.

(2) Preparation of Second Polyolefin Solution

A second polyolefin solution is prepared in the same manner as aboveexcept as follows. A second polyolefin composition comprising (a) 47% ofPE1 having a weight average molecular weight of 3.0×10⁵ and a molecularweight distribution of 8.6, and (b) 3% of PE2 having a weight averagemolecular weight of 2.0×10⁶ and a molecular weight distribution of 8,and (c) 50% of second polypropylene resin having a weight averagemolecular weight of 1.40×10⁶, a heat of fusion of 111.6 J/g, a fractionhaving a molecular weight of 1.8×10⁶ or more with the fraction being25.3% and a molecular weight distribution of 2.6, percentages by weightof the second polyolefin composition, is prepared by dry-blending. Thepolyethylene resin in the composition has a melting point of 135° C. anda crystal dispersion temperature of 100° C. Thirty-five parts by weightof the resultant second polyolefin composition is charged into astrong-blending double-screw extruder having an inner diameter of 58 mmand L/D of 42, and 65 parts by mass of liquid paraffin (50 cst at 40°C.) is supplied to the double-screw extruder via a side feeder.Melt-blending is conducted at 210° C. and 200 rpm to prepare a secondpolyolefin solution.

(3) Production of Membrane

The first and second polyolefin solutions are supplied from theirrespective double-screw extruders to a three-layer-extruding T-die, andextruded therefrom to form an extrudate (also called a laminate) offirst polyolefin solution layer/second polyolefin solution layer/firstpolyolefin solution layer at a layer thickness ratio of 42.5/10/42.5.The extrudate is cooled while passing through cooling rollers controlledat 20° C., to form a three-layer gel-like sheet, which is simultaneouslybiaxially stretched at 118° C. to a magnification of 5 fold in bothmachine (longitudinal) and transverse directions by a tenter-stretchingmachine. The stretched three-layer gel-like sheet is fixed to analuminum frame of 20 cm×20 cm, immersed in a bath of methylene chloridecontrolled at 25° C. to remove liquid paraffin with vibration of 100 rpmfor 3 minutes, and dried by air flow at room temperature. The driedmembrane is re-stretched by a batch-stretching machine to amagnification of 1.4 fold in a transverse direction at 125° C. There-stretched membrane, which remains fixed to the batch-stretchingmachine, is heat-set at 125° C. for 10 minutes to produce a three-layermicroporous membrane.

Example 2

Example 1 is repeated except no re-stretching of the dried three-layermembrane.

Example 3

Example 1 is repeated except the first and second microporous polyolefinmembranes were laminated in an order of first microporousmembrane/second microporous membrane/first microporous membrane at alayer thickness ratio of 25/50/25.

Example 4

Example 1 is repeated except the first polypropylene resin in the firstpolyolefin composition has a weight average molecular weight of 6.6×10⁵,a heat of fusion of 103.3 J/g, a fraction having a molecular weight of1.8×10⁶ or more with the fraction being 8.2% and a molecular weightdistribution of 11.

Example 5

Example 1 is repeated except the first polypropylene resin in the firstpolyolefin composition has a weight average molecular weight of 6.8×10⁵,a heat of fusion of 94.6 J/g, a fraction having a molecular weight of1.8×10⁶ or more with the fraction being 4.7% and a molecular weightdistribution of 5.9.

Example 6

Example 1 is repeated except the first polypropylene resin in the firstpolyolefin composition has a weight average molecular weight of 3.0×10⁵,a heat of fusion of 88.9 J/g, a fraction having a molecular weight of1.8×10⁶ or more with the fraction being 0% and a molecular weightdistribution of 4.9.

Example 7

Example 1 is repeated except the second polypropylene resin in thesecond polyolefin composition has a weight average molecular weight of0.90×10⁶, a heat of fusion of 109.7 J/g, a fraction having a molecularweight of 1.8×10⁶ or more with the fraction being 10.8% and a molecularweight distribution of 2.4.

Example 8

Example 1 is repeated except the second polypropylene resin in thesecond polyolefin composition has a weight average molecular weight of2.69×10⁶, a heat of fusion of 99.9 J/g, a fraction having a molecularweight of 1.8×10⁶ or more with the fraction being 57.2% and a molecularweight distribution of 3.8.

Example 9

Example 1 is repeated except the first polyolefin composition of thefirst polyolefin solution comprises 90% of PE1 and 10% of firstpolypropylene resin, percentages by weight of the first polyolefincomposition. There is no second polyethylene resin in this firstpolyolefin composition.

Example 10

Example 1 is repeated except the second polyolefin composition of thesecond polyolefin solution comprises 50% of PE1 and 50% of firstpolypropylene resin, percentages by weight of the first polyolefincomposition. There is no second polyethylene resin in this fsecondpolyolefin composition.

Comparative Example 1

Example 1 is repeated except the first polyolefin composition of thefirst polyolefin solution comprises 82% first polyethylene resin and 18%second polyethylene resin, no added first polypropylene resin,percentages by weight of the first polyolefin composition,

Comparative Example 2

Example 1 is repeated except the first polyolefin composition of thefirst polyolefin solution. There is no first polyolefin composition.

Comparative Example 3

Comparative Example 1 is repeated except the first polypropylene resinin the first polyolefin composition has a weight average molecularweight of 6.8×10⁵, a heat of fusion of 94.6 J/g, a fraction having amolecular weight of 1.8×10⁶ or more with the fraction being 4.7% and amolecular weight distribution of 5.9, and the second polypropylene resinin the second polyolefin composition has a weight average molecularweight of 6.8×10⁵, a heat of fusion of 94.6 J/g, a fraction having amolecular weight of 1.8×10⁶ or more with the fraction being 4.7% and amolecular weight distribution of 5.9.

Comparative Example 4

Example 1 is repeated except the first polypropylene resin in the firstpolyolefin composition has a weight average molecular weight of1.56×10⁶, a heat of fusion of 78.4 J/g, a fraction having a molecularweight of 1.8×10⁶ or more with the fraction being 35.4% and a molecularweight distribution of 3.2, and the second polypropylene resin in thesecond polyolefin composition has a weight average molecular weight of1.56×10⁶, a heat of fusion of 78.4 J/g, a fraction having a molecularweight of 1.8×10⁶ or more with the fraction being 35.4% and a molecularweight distribution of 3.2.

Comparative Example 5

Comparative Example 1 is repeated except the second polyolefincomposition of the second polyolefin solution. There is no secondpolyolefin composition.

Properties

The properties of the multi-layer microporous membranes of Examples 1-6and Comparative Examples 1-8 are measured by the following methods. Theresults are shown in Tables 1 and 2.

(1) Average Thickness (μm)

The thickness of each microporous membrane is measured by a contactthickness meter at 10 mm intervals in the area of 10 cm×10 cm of themembrane, and averaged. The thickness meter used is a Litematic made byMitsutoyo Corporation.

(2) Standard Deviation of Thickness (μm)

The thickness of each microporous membrane is measured as describedabove. The standard deviation of thickness is calculated based on thethickness data.

(3) Air Permeability (sec/100 cm³/20 μm)

Air permeability P₁ measured on each microporous membrane having athickness T₁ according to JIS P8117 is converted to air permeability P₂at a thickness of 20 μm by the equation of P₂=(P₁×20)/T₁.

(4) Porosity (%)

Measured by a weight method using the formula: Porosity%=100×(w2−w1)/w2, wherein “w1” is the actual weight of film and “w2” isthe assumed weight of 100% polyethylene.

(5) Pin Puncture Strength (mN/20 μm)

The maximum load is measured when each microporous membrane having athickness of T₁ is pricked with a needle of 1 mm in diameter with aspherical end surface (radius R of curvature: 0.5 mm) at a speed of 2mm/second. The measured maximum load L₁ is converted to the maximum loadL₂ at a thickness of 20 μm by the equation of L₂=(L₁×20)/T₁, and used aspin puncture strength.

(6) Thickness Variation Ratio after Heat Compression (%)

A microporous membrane sample is situated between a pair of highly flatplates, and heat-compressed by a press machine under a pressure of 2.2MPa (22 kgf/cm²) at 90° C. for 5 minutes, to determine an averagethickness in the same manner as above. A thickness variation ratio iscalculated by the formula of (average thickness aftercompression−average thickness before compression)/(average thicknessbefore compression)×100, which can be expressed as an absolute value.

(7) Air Permeability after Heat Compression (sec/100 cm³)

Each multi-layer microporous membrane having a thickness of T₁ isheat-compressed under the above conditions, and measured with respect toair permeability P₁ according to JIS P8117.

(8) Electrolytic Solution Absorption Speed

Using a dynamic surface tension measuring apparatus (DCAT21 withhigh-precision electronic balance, available from Eiko Instruments Co.,ltd.), a multi-layer microporous membrane sample is immersed in anelectrolytic solution (electrolyte: 1 mol/L of LiPF₆, solvent: ethylenecarbonate/dimethyl carbonate at a volume ratio of 3/7) kept at 18° C.,to determine an electrolytic solution absorption speed by the formula of[weight increment (g) of microporous membrane/weight (g) of microporousmembrane before absorption]. The electrolytic solution absorption speedis expressed by a relative value, assuming that the electrolyticsolution absorption rate in the microporous membrane of ComparativeExample 5 is 1.

(9) Shut Down Temperature (° C.)

The shut down temperature is measured as follows: A rectangular sampleof 3 mm×50 mm is cut out of the microporous membrane such that thelongitudinal direction of the sample is aligned with the transversedirection of the microporous membrane, and set in a thermomechanicalanalyzer (TMA/SS6000 available from Seiko Instruments, Inc.) at a chuckdistance of 10 mm. With a load of 19.6 mN applied to a lower end of thesample, the temperature is elevated at a rate of 5° C./minute to measureits size change. A temperature at a point of inflection observed nearthe melting point is defined as the shutdown temperature.

(10) Melt down temperature (° C.)

The melt down temperature is measured by using thermomechanical analyzer(TMA/SS6000 available from Seiko Instruments, Inc.) as well as the shutdown temperature above. Melt down temperature is the temperature atwhich the membrane breaks.

(11) Capacity Recovery Ratio

The capacity recovery ratio of a lithium ion battery containing themulti-layer microporous membrane as a separator is measured as follows:First, the discharge capacity (initial capacity) of the lithium ionbattery is measured by a charge/discharge tester before high temperaturestorage. After being stored at a temperature of 80° C. for 30 days, thedischarge capacity is measured again by the same method to obtain thecapacity after high temperature storage. The capacity recovery ratio (%)of the battery is determined by the following equation: Capacityrecovery ratio (%)=[(capacity after high temperature storage)/(initialcapacity)]×100.

TABLE 1 No Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Resin composition FirstPolyolefin PE1 Mw 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0× 10⁵ Mw/Mn 8.6 8.6 8.6 8.6 8.6 8.6 % by mass 82 82 82 82 82 82 PE2 Mw2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ Mw/Mn 8 8 88 8 8 % by mass 8 8 8 8 8 8 PP Mw 1.40 × 10⁶  1.40 × 10⁶  1.40 × 10⁶ 6.6 × 10⁵ 6.8 × 10⁵ 3.0 × 10⁵ Mw/Mn 2.6 2.6 2.6 11 5.9 4.9 HMWF⁽¹⁾ 25.325.3 25.3 8.2 4.7 0 Heat of fusion (J/g) 111.6 111.6 111.6 103.3 94.688.9 % by mass 10 10 10 10 10 10 Conc. of PO Comp. % by mass 25 25 25 2525 25 Second Polyolefin PE1 Mw 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵3.0 × 10⁵ 3.0 × 10⁵ Mw/Mn 8.6 8.6 8.6 8.6 8.6 8.6 % by mass 47 47 47 4747 47 PE2 Mw 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶Mw/Mn 8 8 8 8 8 8 % by mass 3 3 3 3 3 3 PP Mw 1.40 × 10⁶  1.40 × 10⁶ 1.40 × 10⁶  1.40 × 10⁶  1.40 × 10⁶  1.40 × 10⁶  Mw/Mn 2.6 2.6 2.6 2.62.6 2.6 HMWF⁽¹⁾ 25.3 25.3 25.3 25.3 25.3 25.3 Heat of fusion (J/g) 111.6111.6 111.6 111.6 111.6 111.6 % by mass 50 50 50 50 50 50 Conc. of POComp. % by mass 35 35 35 35 35 35 Production condition Extrudate Layerstructure⁽²⁾ (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I)(I)/(II)/(I) (I)/(II)/(I) Layer thickness ration 40/20/40 40/20/4025/50/25 40/20/40 40/20/40 40/20/40 Stretching of Gel-Like sheetTemperature (° C.) 118 118 118 118 118 118 Magnification (MD × TD)⁽³⁾ 5× 5 5 × 5 5 × 5 5 × 5 5 × 5 5 × 5 Stretching of dried membraneTemperature (° C.) 125 — 125 125 125 125 Magnification (TD) 1.4 — 1.41.4 1.4 1.4 Heat setting treatment Temperature (° C.) 125 125 125 125125 125 Time (min) 10 10 10 10 10 10 Properties Average thickness (μm)19.9 21.0 20.0 20.5 20.4 20.4 STDEV of thickness (μm) 0.61 0.66 0.592.11 2.49 3.58 Air Perm. (sec/100 cm³/20 μm) 220 315 502 203 215 194Porosity % 51.9 49.9 48.2 53.2 53.4 52.6 Punct. Strength (mN/20 μm) 31503010 2930 3050 3000 2950 Thick. Var. Aft. Heat Comp. % −10 −13 −11 −9−10 −12 Air Perm. Aft. Heat Comp. 435 630 980 410 430 400 Elec. Soln.Absorp. Speed 3.4 2.1 3.6 2.2 2.1 1.9 Shut Down Temp. ° C. 135 135 135135 135 135 Melt Down Temp. ° C. 178 178 180 177 177 176 Cap. RecoveryRatio % 78 77 78 77 76 70 No Ex 7 Ex 8 Ex 9 Ex 10 Resin compositionFirst Polyolefin PE1 Mw 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ 3.0 × 10⁵ Mw/Mn8.6 8.6 8.6 8.6 % by mass 82 82 90 82 PE2 Mw 2.0 × 10⁶ 2.0 × 10⁶ — 2.0 ×10⁶ Mw/Mn 8 8 — 8 % by mass 8 8 — 8 PP Mw 1.40 × 10⁶  1.40 × 10⁶  1.40 ×10⁶  1.40 × 10⁶  Mw/Mn 2.6 2.6 2.6 2.6 HMWF⁽¹⁾ 25.3 25.3 25.3 25.3 Heatof fusion (J/g) 111.6 111.6 111.6 111.6 % by mass 10 10 10 10 Conc. ofPO Comp. % by mass 25 25 25 25 Second Polyolefin PE1 Mw 3.0 × 10⁵ 3.0 ×10⁵ 3.0 × 10⁵ 3.0 × 10⁵ Mw/Mn 8.6 8.6 8.6 8.6 % by mass 47 47 47 50 PE2Mw 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ — Mw/Mn 8 8 8 — % by mass 3 3 3 — PP Mw0.90 × 10⁶  2.69 × 10⁶  1.40 × 10⁶  1.40 × 10⁶  Mw/Mn 2.4 3.8 2.6 2.6HMWF⁽¹⁾ 10.8 57.2 25.3 25.3 Heat of fusion (J/g) 109.7 99.9 111.6 111.6% by mass 50 50 50 50 Conc. of PO Comp. % by mass 35 35 35 35 Productioncondition Extrudate Layer structure⁽²⁾ (I)/(II)/(I) (I)/(II)/(I)(I)/(II)/(I) (I)/(II)/(I) Layer thickness ration 40/20/40 40/20/4040/20/40 40/20/40 Stretching of Gel-Like sheet Temperature (° C.) 118118 118 118 Magnification (MD × TD)⁽³⁾ 5 × 5 5 × 5 5 × 5 5 × 5Stretching of dried membrane Temperature (° C.) 125 125 125 125Magnification (TD) 1.4 1.4 1.4 1.4 Heat setting treatment Temperature (°C.) 125 125 125 125 Time (min) 10 10 10 10 Properties Average thickness(μm) 19.3 19.9 19.5 20.2 STDEV of thickness (μm) 0.61 0.60 0.53 0.60 AirPerm. (sec/100 cm³/20 μm) 188 330 231 208 Porosity % 53.1 48.2 52.4 52.1Punct. Strength (mN/20 μm) 3100 3300 3210 3220 Thick. Var. Aft. HeatComp. % −8 −7 −10 −12 Air Perm. Aft. Heat Comp. 389 660 480 440 Elec.Soln. Absorp. Speed 3.5 2.9 3.5 3.3 Shut Down Temp. ° C. 135 135 135 135Melt Down Temp. ° C. 175 176 177 178 Cap. Recovery Ratio % 78 77 81 80

TABLE 2 No Comp. Ex 1 Comp. Ex 2 Comp. Ex 3 Comp. Ex 4 Comp. Ex 5 Resincomposition First Polyolefin PE1 Mw 3.0 × 10⁵ — 3.0 × 10⁵ 3.0 × 10⁵ 3.0× 10⁵ Mw/Mn 8.6 — 8.6 8.6 8.6 % by mass 82 — 82 82 82 PE2 Mw 2.0 × 10⁶ —2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ Mw/Mn 8 — 8 8 8 % by mass 18 — 8 8 18 PPMw — — 6.8 × 10⁵ 1.56 × 10⁶  — Mw/Mn — — 5.9 3.2 — HMWF⁽¹⁾ — — 8.4 1.2 —Heat of fusion (J/g) — — 94.6 78.4 — % by mass — — 10 10 — Conc. of POComp. % by mass 25 — 25 25 25 Second Polyolefin PE1 Mw 3.0 × 10⁵ 3.0 ×10⁵ 3.0 × 10⁵ 3.0 × 10⁵ — Mw/Mn 8.6 8.6 8.6 8.6 — % by mass 47 47 47 47— PE2 Mw 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ 2.0 × 10⁶ — Mw/Mn 8 8 8 8 — % bymass 3 3 3 3 — PP Mw 1.40 × 10⁶  1.40 × 10⁶  6.8 × 10⁵ 1.56 × 10⁶  —Mw/Mn 2.6 2.6 5.9 3.2 — HMWF⁽¹⁾ 25.3 25.3 8.4 1.2 — Heat of fusion (J/g)111.6. 111.6 94.6 78.4 — % by mass 50 50 50 50 — Conc. of PO Comp. % bymass 35 35 35 35 — Production condition Extrudate Layer structure⁽²⁾(I)/(II)/(I) (II) (I)/(II)/(I) (I)/(II)/(I) (I) Layer thickness ration40/20/40 100 40/20/40 40/20/40 100 Stretching of Gel-Like sheetTemperature (° C.) 118 118 118 118 115 Magnification (MD × TD)⁽³⁾ 5 × 55 × 5 5 × 5 5 × 5 5 × 5 Stretching of dried membrane Temperature (° C.)125 125 125 125 125 Magnification (TD) 1.4 1.4 1.4 1.4 1.4 Heat settingtreatment Temperature (° C.) 125 125 125 125 127 Time (min) 10 10 10 1010 Properties Average thickness (μm) 20.3 20.2 19.1 19.6 20.3 STDEV ofthickness (μm) 0.39 0.54 2.33 1.13 0.41 Air Perm. (sec/100 cm³/20 μm)360 430 100 690 370 Porosity % 47.8 45.2 58.6 48.2 39 Punct. Strength(mN/20 μm) 3210 3420 1710 1280 4410 Thick. Var. Aft. Heat Comp. % −20−11 −17 −20 −21 Air Perm. Aft. Heat Comp. 880 820 210 1650 830 Elec.Soln. Absorp. Speed 1.2 2.4 2.7 1.6 1 Shut Down Temp. ° C. 135 135 135135 135 Melt Down Temp. ° C. 177 179 162 160 148 Cap. Recovery Ratio %67 81 73. 73 65 ⁽¹⁾HMWF represents a low-molecular-weight fractionhaving a molecular weight of 1.8 × 10⁶ or more (% by mass). ⁽²⁾(I)represents the first polyolefin solution, and (II) represents the secondpolyolefin solution ⁽³⁾(MD × TD) represents the magnification in alongitudinal direction (MD) and a transverse direction (TD).

It is noted from Table 1 that the multi-layer microporous membrane ofthe present invention has well-balanced properties, including standarddeviation of thickness, air permeability, pin puncture strength, shutdown temperature and melt down temperature, as well as excellentelectrolytic solution absorption, with little variation of thickness andair permeability after heat compression. Lithium ion secondary batteriescomprising the multi-layer microporous membranes of the presentinvention have capacity recovery ratios of 70% or more, indicatingdesirable high temperature retention properties.

On the other hand, the microporous membrane products of the ComparativeExamples exhibit a poorer balance of properties.

The multi-layer microporous membrane of the present invention haswell-balanced properties and use of such multi-layer microporousmembrane as a battery separator provides batteries having excellentsafety, heat resistance, retention properties and productivity.

What is claimed is:
 1. A multi-layer microporous membrane, comprising: afirst layer material comprising a first polyethylene and a firstpolypropylene having a weight-average molecular weight of 3.0×10⁵ to1.40×10⁶ and a second layer material comprising a second polyethyleneand a second polypropylene consisting of a propylene homopolymer, thesecond polypropylene having a weight-average molecular weight of0.90×10⁶ to 2.69×10⁶ and a heat of fusion of 95 J/g or more the fractionof the second polypropylene having a molecular weight of 1.8×10⁶ or morebeing 10% or more by mass of the second polypropylene and the first andsecond polypropylenes have different weight-average molecular weights.2. The multi-layer microporous membrane of claim 1, wherein themulti-layer microporous membrane comprises a first layer containing thefirst layer material and a second microporous layer containing thesecond layer material, and at least one of the first layer material orthe second layer material is present in a skin layer of the multi-layermicroporous membrane.
 3. The multi-layer microporous membrane of claim1, wherein the multi-layer membrane comprises: a first microporous layercontaining the first layer material, a third microporous layercontaining the first layer material, and a second microporous layercontaining the second layer material, the second microporous layer beinglocated between the first and third microporous layers.
 4. Themulti-layer microporous membrane of claim 1, wherein the multi-layermicroporous membrane comprises: a first microporous layer containing thesecond layer material, a third microporous layer containing the secondlayer material, and a second microporous layer containing the firstlayer material, the second microporous layer being located between thefirst and third microporous layers.
 5. The multi-layer microporousmembrane of claim 3, wherein the first polyethylene is present in thefirst layer material in a first polyethylene amount in the range of fromabout 50 wt. % to about 99 wt. % based on the weight of the first layermaterial, the first polypropylene is present in the first layer materialin a first polypropylene amount in the range of from about 1 wt. % toabout 50 wt. % based on the weight of the first layer material, thesecond polyethylene is present in the second layer material in a secondpolyethylene amount in the range of from about 5 wt. % to about 95 wt. %based on the weight of the second layer material, and the secondpolypropylene is present in the second layer material in a secondpolypropylene amount in the range of from about 5 wt. % to about 95 wt.% based on the weight of the second layer material.
 6. The multi-layermicroporous membrane of claim 5, wherein (a) the first and/or secondpolyethylene comprises a polyethylene having a weight-average molecularweight ranging from about 1×10⁴ to about 5×10⁵, a polyethylene having aweight-average molecular weight of at least about 1×10⁶, or both,wherein (1) the first and/or second polyethylene has a weight-averagemolecular weight in the range of about 1×10⁴ to about 1×10⁷; (2) thefirst and/or second polyethylene has it weight-average molecular weightin the range of about 1×10⁴ to about 5×10⁷; (3) the polyethylene havinga weight-average molecular weight ranging from about 1×10⁴ to about5×10⁵ is one or more of a high-density polyethylene, a medium-densitypolyethylene, a branched low-density polyethylene, or a linearlow-density polyethylene; (4) the polyethylene having a weight-averagemolecular weight ranging from about 1×10⁴ to about 5×10⁵ is at least oneof (i) an ethylene homopolymer or (ii) a copolymer of ethylene and athird a-olefin selected from the group of propylene, butene-1, hexene-1;(5) the polyethylene having a weight-average molecular weight of atleast 1×10⁶ has a weight-average molecular weight of at least about1×10⁶; (6) the polyethylene having a weight-average molecular weight ofat least 1×10⁶ is at least one of (i) an ethylene homopolymer or (ii) acopolymer of ethylene and a fourth α-olefin selected from the group ofpropylene, butene-1, hexene-1; (7) the amount of the polyethylene havinga weight-average molecular weight ranging from about 1×10⁴ to about5×10⁵ in the first microporous layer material is in the range of fromabout 70 wt. % to about 90 wt. %, based on the weight of the firstmicroporous layer material; (8) the amount of the polyethylene having aweight-average molecular weight of at least about 1×10⁶ in the firstmicroporous layer material is in the range of from, about 0 wt. % toabout 10 wt. %, based on the weight of the first microporous layermaterial; (9) the amount of the polyethylene having a weight-averagemolecular weight ranging from about 1×10⁴ to about 5×10⁵ in the secondmicroporous layer material is in the range of from about 40 wt. % toabout 60 wt. %, based on the weight of the second microporous layermaterial; (10) the amount of the polyethylene having a weight-averagemolecular weight of at least about 1×10⁶ in the second microporous layermaterial is in the range of from about 0 wt. % to about 10 wt. %, basedon the weight of the second microporous layer material; (11) the firstand/or second polyethylene has a molecular weight distribution (“Mw/Mn”)of about 5 to about 300; (b) the first polypropylene has at least onecharacteristic selected from: (1) the first polypropylene is one or moreof (i) a propylene homopolymer or (ii) a copolymer of propylene and afifth olefins selected from one or more of α-olefins such as ethylene,butene-1, pen hexane-1,4-methylpentene-1, octene-1, vinyl acetate,methyl methacrylate, styrene, butadiene, 1,5-hexadiene, 1,7-ortadieneand 1,9-decadiene; (2) the first polypropylene has an Mw/Mn ranging fromabout 1.01 to about 100; (3) the first polypropylene is isotactic; (4)the first polypropylene has a heat of fusion of at least about 90Joules/gram; (5) the first polypropylene has a melting peak (secondmelt) of at least about 160° C.; and (c) the second polypropylene has anMw/Mn of 2.5 or less.
 7. The multi-layer microporous membrane of claim1, wherein the first and/or second polyethylene has a weight-averagemolecular weight in the range of about 2×10⁵ to about 3×10⁶.
 8. Themulti-layer microporous membrane of claim 1, wherein the first and/orsecond polyethylene has a weight-average molecular weight in the rangeof 1×10⁴ to 5×10⁵.
 9. The multi-layer microporous membrane of claim 6,wherein the polyethylene having a weight-average molecular weightranging from about 1×10⁴ to about 5×10⁵ is high-density polyethylene andthe polyethylene having a weight-average molecular weight of at leastabout 1×10⁶ is ultra-high molecular weight polyethylene.
 10. Themulti-layer membrane of claim 6, wherein the first and/or secondpolyethylene comprises 10 wt. % or less of the polyethylene having aweight-average molecular weight of at least about 1×10⁶ and 90 wt. % ormore of the polyethylene having a weight-average molecular weightranging from about 1×10⁴ to about 5×10⁵.
 11. A battery comprising ananode, a cathode, an electrolyte, and the multi-layer microporousmembrane of claim 1, wherein the multi-layer microporous membraneseparates at least the anode from the cathode.
 12. The battery of claim11, wherein the electrolyte contains lithium ions and the battery is asecondary battery.
 13. The battery of claim 11, wherein the multi-layermembrane has a porosity of 25 to 80%, an air permeability of 20 to 700seconds/100 cc (converted to the value at 20-μm thickness), a pinpuncture strength of 2,000 mN/20 μm or more, a shut down temperature of120 to 140° C., and a meltdown temperature of 170° C. or higher, and abattery capacity recovery ratio of 70% or more.
 14. The battery of claim11 used as a source or sink of electric charge.